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Heat the olive oil in a 3.5 quart skillet or Dutch oven over medium heat. Sauté the onion and pepper until softened, about 8 minutes.
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Add in the garlic, cumin, chili powder, and cayenne pepper, if using, and sauté until fragrant, about one more minute.
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Add in the chopped tomatoes (with their juices), salt, quinoa, red lentils, water, and black beans. Stir well and bring the liquid to a boil. Cover and lower the heat to a simmer, cooking until the quinoa and lentils are tender, about 15 minutes.
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Remove the lid and fluff with a fork. Serve warm with a topping of fresh cilantro, avocado slices, and radishes, if desired.