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.
Add garlic, diced tomatoes, 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.