1. In a large skillet, heat oil over medium-high heat. Add onion and cook until softened, about 3 minutes. Add ground sirloin and cook, stirring often with a spatula, until meat browns, about 10 minutes; drain excess fat. Stir in 1/2 cup salsa, chili powder, cumin, and pepper. Continue cooking until heated through.
2. Preheat oven or toaster oven to 350°. Put taco shells on a baking sheet and heat as per package directions.
3. Combine beans and remaining 1/2 cup salsa in a small saucepan. Cook over medium heat, stirring occasionally, until heated through, about 3 minutes. Keep warm.
4. To serve, arrange taco shells on platter. Place meat on a separate serving dish, and put the beans, cheese, lettuce, and sour cream in bowls. Let diners assemble their own tacos.