Heat oil in a skillet over medium heat. Add onion and sauté until caramelized.
Add garlic, black beans, lentils, marinara sauce, and spices. Mix well, then lower heat and put a lid on the skillet. Simmer for about 10 minutes, stirring occasionally.
Toast the taco shells to your liking, then stuff with the lentil mixture. Sprinkle on optional toppings, and enjoy!