To prepare a spiced lentil casserole casseroles recipe, you'll first need to prepare the lentils. You can use any type of lentil, as long as they aren't mushy. If you want to use white, brown, or black, make sure they're all mixed well. Then, add the chopped onions, garlic, thyme, and salt and pepper to taste. Cook the mixture over medium-low heat until the onions are translucent, about 5 minutes. Add the diced tomatoes and cook for an additional 5 to 10 minutes, stirring frequently. Remove the vegetables from the heat and stir in the cooked lentil mixture. Serve with sour cream and chopped parsley.