Preheat the oven to 400°F. Place your taco shells on a baking sheet and bake for 2–3 minutes. This makes them extra crunchy.
Make the bean filling: In a skillet, heat the olive oil and sauté the diced onion for 2–3 minutes. Add taco seasoning and black beans. Stir and cook another 2–3 minutes until warm and fragrant.
Assemble your tacos: Spoon 3–4 tablespoons of bean mixture into each taco shell. Top with lettuce, cheese, and a drizzle of salsa.
Add lime juice: Finish each taco with a little fresh lime juice. Add any other toppings you love, then enjoy right away!