French Onion Grilled Cheese (with Caramelized Onion)

French onion grilled cheese is the best of both worlds: French onion soup AND grilled cheese. This way, you don't have to choose one over the other for dinner - it's like a fancier, grown-up grilled cheese sandwich. The caramelized onions are cooked low and slow until golden and sweet. Gruyère and Swiss cheese are added, creating a melty, gooey, and rich sandwich with hints of garlic and thyme.

One of my favorite ways to make a grilled cheese sandwich is by adding caramelized onions and turning it into a French onion grilled cheese (very similar to a British cheese and onion toastie).

The caramelized onions are cooked low and slow, and a slight tanginess is provided by white wine vinegar, which helps to cut the richness of Gruyère and creamy Swiss cheese.

Tangy sourdough marries all of the flavors and holds up well against this delightfully gooey mess of a sandwich. Serve it with vegetarian white bean chili or cucumber gazpacho for a hearty meal!

Jump to:

• ⭐️ Why This Grilled Cheese Recipe is the Best
• 🤔 What is the Secret to the Best Grilled Cheese?
• 🌿 Ingredient Notes
• 📝 Variations & Substitutions
• 🔪 How To Make Caramelized Onion Grilled Cheese
• 👩🏼‍🍳 Expert Tips
• 🍽 How to Serve Grilled Cheese with Caramelized Onions
• 💬 Frequently Asked Questions
• 🥪 More Sandwich Recipes
• French Onion Grilled Cheese (with Caramelized Onion)

⭐️ Why This Grilled Cheese Recipe is the Best

• Melty, gooey, rich, and toasty sourdough grilled cheese
• Easy weeknight dinner or more indulgent weekend treat
• Vegetarian comfort food
• Budget-friendly recipe
• Cheesy, buttery, slightly sweet, and tangy French onion soup grilled cheese

🤔 What is the Secret to the Best Grilled Cheese?

When it comes to making the best grilled cheese sandwich, it's all about achieving that perfect balance of crispy, golden-brown exterior and gooey, melty interior.

First, choose a high-quality, slightly thick-sliced bread. Next, use a generous amount of butter on each slice, as that's what gives it that coveted crunch and rich flavor. Another key is using two different types of cheese. This can take your grilled cheese to the next level.

Finally, don't rush the cooking process. Slow and steady wins the race here! Cook your sandwich over medium-low heat, giving it enough time to become beautifully golden on the outside while allowing the cheese to melt into a creamy, luscious filling. Remember, patience is key to mastering the art of the best grilled cheese.

🌿 Ingredient Notes

• Unsalted butter - Smearing your bread with butter makes for a golden (and much richer) grilled cheese. I recommend using unsalted butter to control the amount of salt used. If you only have salted butter on hand, skip the added salt.
• Yellow onion - The star of this sandwich. Make sure you go low and slow when caramelizing your onion. If you love caramelized onions like I do, don't miss my French onion dip and caramelized shallot pasta!
• Fresh thyme leaves and garlic - Adds lovely aromatics to our grilled cheese with onions.
• White wine vinegar - Cuts through the richness of the cheese and adds a slight zippy note.
• Gruyère cheese - A nutty, salty, and earthy cheese that's key to French onion soup. Make sure to avoid pre-grated cheese, as it will not melt as well as a block of cheese that you grate yourself.
• Swiss cheese - A milder, softer cheese that melts really well.
• Sourdough bread - Use high-quality sourdough (or your favorite loaf of bread) for this cheese and onion sandwich. It really makes a difference.

📝 Variations & Substitutions

• Use red onion for a slightly different flavor profile.
• Vinegar. Substituting red wine vinegar or apple cider vinegar will work nicely in place of the white wine vinegar.
• Use mayonnaise in place of butter to slather your bread before toasting. It may sound strange, but some people swear by it! Using mayo adds more tanginess and richness to your grilled cheese sandwich.

🔪 How To Make Caramelized Onion Grilled Cheese

Caramelize onions: Over medium heat, melt 2 tablespoons of butter in a large frying pan. Add sliced onion and adjust heat to medium-low. Cook until onion is soft and brown, around 30-45 minutes, stirring every once in a while.

Add thyme, garlic, salt, and white wine vinegar, and cook for an additional 2 minutes. Transfer onions to a bowl.

Use a paper towel to wipe out the skillet. Then, adjust heat to low and add remaining 2 tablespoons of butter to pan. Place two slices of sourdough in pan, and distribute cheese evenly over both slices of sourdough. Add caramelized onion mixture and place remaining 2 slices of sourdough on top.

Cook on one side until golden brown, 5-6 minutes, then flip and cook other side for another 3-4 minutes. Remove from pan and cut in half. Serve immediately.

👩🏼‍🍳 Expert Tips

• Go low and slow. When caramelizing onions, be sure to use a low heat setting. You don't want to fry the onions, but instead transform them into silky, soft onions that become slightly sweet.
• Use high-quality bread. I love the slight tang of sourdough, but use whatever your favorite loaf of bread is. Treat yourself to a nice loaf from your local bakery - it's worth it for this toasted sandwich.
• How to store: Wrap grilled cheese in foil and store in the refrigerator for up to 4 days; follow reheating instructions below.
• To reheat: Use your toaster oven (or regular oven) for best results. I recommend reheating for about 10 minutes at 350 degrees F. Microwaving your sandwich may lead to soggy bread, which would not be pleasant!
• Make in oven: Heat oil or butter in a large ovenproof skillet over medium heat. Toast both sides of bread until golden brown. Place in a 350 degree F. oven until cheese is melted and gooey.
• Avoid pre-grated cheese. Try to avoid pre-grated (shredded) cheese, as it includes additives that prevent it from melting nicely.

🍽 How to Serve Grilled Cheese with Caramelized Onions

Pair your homemade grilled cheese sandwich with a side of vegetables, like roasted carrots and green beans

Serve it with a side salad, like citrus fennel salad or cabbage slaw

Pair it with a hearty cup of soup, like chickpea soup

Serve it with a savory dipping sauce, like Sriracha garlic sauce, for a spicy kick

💬 Frequently Asked Questions

🥪 More Sandwich Recipes

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