For the best flavor, warm the tortillas briefly in the microwave before serving.
In a large nonstick skillet, heat oil over medium. Add onion and bell peppers; cook until crisp-tender, 3 to 5 minutes. Add 1 cup water; cook until water has evaporated and vegetables are softened, 3 to 5 minutes.2.
Add garlic, tomato paste, chili powder, and cumin; season with salt and pepper. Cook, stirring, 1 minute. Add beef. Cook, breaking up meat with a wooden spoon, until no longer pink, about 5 minutes. Season taco filling with salt and pepper. To serve, spoon filling onto tortillas, top with salsa and lettuce, and fold to enclose.