Fajita Stir-Fry
["1 lb. lean boneless beef steak", "2 Tbsp. oil", "2 cloves garlic, minced", "1 large onion, sliced and separated", "3 large jalapeno chilies, minced", "1 large bell pepper, sliced", "2 tsp. ground cumin", "3 Tbsp. lime juice", "1 tsp. cornstarch", "2 medium Roma-type tomatoes, diced", "salt and pepper", "lime wedges (optional)", "sour cream (optional)", "1 large ripe avocado, peeled and diced", "8 warm flour tortillas (8-inch diameter) *", "homemade or purchased salsa"]
["*Flour tortillas may be replaced with large iceberg lettuce leaves, chilled."]
["beef steak", "oil", "garlic", "onion", "jalapeno chilies", "bell pepper", "ground cumin", "lime juice", "cornstarch", "tomatoes", "salt", "lime wedges", "sour cream", "avocado", "flour tortillas", "salsa"]