Heat olive oil in a pot over medium heat.
Stir in the onion and garlic and cook until the onion is softened and fragrant, about 5 minutes.
Pour in the water, lentils, brown rice, chili powder and cumin.
Bring mixture to a boil.
Lower heat to a simmer and cook until rice and lentils are tender and water is absorbed, about 45 minutes.
Stir salt into lentil and brown rice mixture.
Scoop a spoonful or two of lentils and rice into a small tortilla.
Top with your favorite taco toppings, cheese, diced tomatoes, cilantro, etc.