Cut tops off peppers, remove core and seeds, and place in a baking dish. If the peppers don’t stand up in the dish, slice a little off the bottom, without cutting through the pepper, to stabilize. Chop the pepper tops into pieces.
Cook and cool rice and lentils according to package directions. Sauté celery, chopped pepper tops, onions and mushrooms with salt, pepper and blackened seasoning in a little olive oil.
Preheat oven to 350 degrees. Bake unfilled peppers for 10 minutes and remove from oven.
Combine cooled rice, lentils and cooked vegetables in a bowl and add shredded carrots and tomatoes. Add extra blackened seasoning, salt and pepper to taste. Stir in lemon juice, to taste, for a little acidity. Stuff each pepper. (Depending on the size of the peppers, you may have extra filling, and it is delicious by itself.)
Bake uncovered at 350 degrees for 10-20 minutes, until the peppers are softened.