Wash the cored peppers, and tops if using them. Lightly salt them if desired. Place the bottoms of the peppers in a large crockpot.
Heat a saute pan to medium-high. Add the olive oil, onions, garlic, and turkey. Cook until the onion is softened and the turkey is no longer pink.
Add the basil, fennel seeds (to taste),salt, tomatoes, and Worcestershire sauce. Stir well to combine. Taste the sauce, and if it is too acidic, add up to 1 teaspoon of sugar.
Add the lentils. Stir well to combine everything, and bring the mixture to a simmer.
Let the meat, lentil mixture simmer for about 15 minutes, and then stir in half of the feta cheese.
Spoon the meat-lentil filling into the bell peppers. Sprinkle the remaining feta cheese over the top of each pepper and place the tops on the peppers if using them.
Cover the crockpot and let the peppers cook on low for about 3 hours, or until the peppers are tender.
Cool any leftover peppers and place them in freezer bags or plastic containers to store in the freezer for up to 2 months.
NOTE: if you prefer to bake these in a casserole dish or baking pan, just put about 1/2 inch water in the bottom of the pan, cover the container with a lid or foil, and bake them for about 1 hour.