Place all ingredients in a 6-quart slow cooker. Cook on low heat for 8 hours. When the chili is finished, stir in a little extra water if it's too thick for your liking. Serve.
To prepare the chili over the stove, heat a tablespoon oil in a large soup pot over medium heat. Add the onions and peppers and sauté for five minutes, till the onions are clear and soft. Add the garlic and sauté the ingredients for another two minutes, adding a tablespoon or two of water if the mixture starts to stick to the bottom of the pot. Add all remaining ingredients and bring the chili to a boil. Cover, reduce heat to low, and simmer for 30-35 minutes, or until all of the lentils are tender. Serve.
Chili leftovers will keep for up to 5 days in an airtight container in the fridge. They can be frozen for up to 1 month.