In a small bowl, whisk together the butter, garlic, and parsley (if using). Set aside.
Heat a large skillet over medium heat. Spread garlic butter on one side of each slice of bread.
Place two buttered bread slices (buttered side down) in skillet. Place 1-2 slices of cheddar, provolone, and tomato, along with pinch of salt and pepper, on top of bread, and top with remaining buttered sourdough (with buttered side facing up).
Cook for around 4-5 minutes or until bottom turns golden brown. Flip over sandwich and cook another 4-5 minutes, until other side is golden brown. Serve warm.
Go low and slow. Be patient with your grilled cheese. If it looks like your cheese isn't melting after a few minutes, cover the pan for a minute or two to help it along.To reheat: I recommend reheating your grilled cheese in the toaster oven (or regular oven) to help re-crisp the bread. Place it in a 350 degree F oven for around 10 minutes. Reheating in the microwave may lead to soggy bread.