In a medium saucepan, heat 2 teaspoons olive oil over medium heat. Add the onion and cook, stirring occasionally, until soft, about 3 minutes. Stir in the garlic, chili powder, cumin, ¼ teaspoon salt, tomato sauce, and water, and cook at a low simmer for 5 minutes. Remove from the heat. Stir in the cooked quinoa, black beans, cilantro, and cheddar cheese.