Bring water and lentils to a boil in a large saucepan. Cover, reduce heat to low, and simmer until mostly tender, 10 to 15 minutes.

Meanwhile, heat a stew pot over low heat. Add oil and carrots; cook, covered, until carrots begin to soften, about 15 minutes. Drain cooked lentils well.

Add onions and mushrooms to the carrots. Season with salt and pepper. Cover and cook for 10 minutes. Add bacon and cook, covered, for 10 minutes. Stir in lentils; cover and simmer until tender, about 15 minutes more.