Cut bell peppers in half, keeping stems intact. Discard seeds and membranes. Cook bell peppers in boiling water 5 minutes; drain and set aside.
Combine 2 cups water, lentils, and salt in a saucepan. Bring to a boil; cover, reduce heat to medium-low, and simmer 20 minutes. Drain and set aside.
Bring 1 cup water to a boil in a medium saucepan; add rice and onion. Cover, reduce heat, and simmer 20 minutes or until liquid is absorbed. Remove from heat. Set aside. Place sausage in a small nonstick skillet over medium-high heat; saute until browned.
Preheat oven to 350°.
Combine lentils, rice mixture, sausage, mozzarella, and remaining ingredients except Parmesan in a large bowl. Divide lentil mixture evenly among bell peppers; sprinkle with Parmesan. Place stuffed peppers in an 11 x 7-inch baking dish; cover and bake at 350° for 15 minutes. Uncover and bake an additional 15 minutes.