Cook the rice as per the packages instructions. If using dry lentils, also cook them now, as per instructions on package, but use vegetable broth instead of water (it's way more flavorful this way).
Pre-heat oven to 425 degrees F.
Wash your peppers, cut them in half, gut them, and set them on a baking tray face down.
Place the peppers in the oven and roast them for 15-20 minutes or until they start to blister. Remove and set aside to cool. (While the peppers roast, proceed with the next step.)
In a large skillet, heat oil over moderate heat. Add the garlic and onions, and sauté until the garlic is fragrant and the onions are translucent (about 5 minutes). Stir often and care not to brown the garlic, or it will result in a bitter taste.
Add the mushrooms and tomatoes; season with salt and pepper, and sauté until the mushrooms soften (about 5 minutes).
Finally, add the lentils, rice, black olives, tomato sauce, and 1 cup of shredded mozzarella cheese. Stir well, bring the heat to low, and simmer for about 5 minutes.
Set the broiler to high, and set oven rack one space higher than the middle (this can depend on your oven, so use your best judgement).
Remove the skillet from the heat and, with a large spoon, ladle the stuffing into the open roasted peppers. Stuff them to the brim!
Sprinkle each pepper with the remaining mozzarella cheese, and grate some fresh parmesan or pecorino romano on there if you'd like.
Place the peppers in the oven and broil until the cheese starts to melt and turn a nice light golden color (approx. 3-5 minutes, but it depends on your oven, so keep a close eye on it.)
When done, remove the peppers from the oven and garnish with freshly chopped herbs.