Meanwhile, add liquid and lentils to a small saucepan. Over medium-high heat, bring to a boil, then reduce heat to a simmer and cook (covered) for around 20 minutes, or until tender. Drain any remaining liquid and set aside.
While your lentils are cooking, add oil to a large skillet over medium heat. Add diced onion and cook 5-6 minutes, until soft and translucent. Toss in diced pepper and mushrooms, and cook for another 3-4 minutes. Add minced garlic and cook for an additional minute.