Preheat oven to 400 degrees F. In a large Dutch oven heat oil, add onions, peppers and celery, and cook over medium heat until soft, about 8 to 10 minutes. Add garlic and cook 2 to 3 minutes more. Stir in chili powder to taste, cumin, oregano, and ginger, and cook 1 to 2 minutes. Add chicken stock, lentils and tomatoes. Bring to the boil, reduce the heat, and simmer for 45 to 50 minutes. Adjust seasoning with salt and pepper. Transfer to an ovenproof serving dish or pie plate. In a mixing bowl stir together flour and cornmeal. Whisk together egg, buttermilk and oil and stir into flour mixture until just blended. Spoon corn bread mixture on top of the chili and bake 25 to 30 minutes. Let casserole stand 10 minutes before cutting into wedges and serving.