Lighter Beef Tacos

Name: Lighter Beef Tacos
Ingredients: 1 teaspoon vegetable oil, such as safflower; 1 large onion, chopped; 2 bell peppers (ribs and seeds removed), chopped; 3 garlic cloves, minced; 2 tablespoons tomato paste; 2 teaspoons chili powder; 1 1/2 teaspoons ground cumin; Coarse salt and ground pepper; 3/4 pound ground beef sirloin; 12 soft corn tortillas (6-inch); Store-bought salsa and shredded iceberg lettuce, for serving