In a Dutch oven or deep, heavy pot, cook the onion in the oil for 10 minutes over low heat. Add the garlic, chili powder, cumin, oregano, thyme, paprika, and pepper flakes; cook, stirring, for 3 minutes. Stir in the chicken broth, lentils, and tomatoes; simmer 40 minutes; add up to 1 cup water if the mixture becomes dry. Add the bell peppers and celery, and simmer for 15 minutes longer. (At this point the chili can be cooled and refrigerated.) Turn the chili into a 3-quart deep casserole (reheat first if refrigerated). Preheat the oven to 400째F. Drop the Corn Bread Crust batter by spoonfuls over the chili. Bake for 30 minutes or until a wooden pick inserted into the corn bread comes out clean.