Preheat an indoor or outdoor grill to medium-high. Brush the steak with 1 Tbsp. of the oil; season with salt and pepper. Cook the steak to the desired doneness. Allow 2 to 3 minutes per side for rare, 3 to 4 minutes for medium-rare, and 4 to 5 minutes for medium. While the steak grills, place the remaining oil in a large, non-stick skillet and set over medium-high heat. Add the onion and bell peppers and cook 2 minutes. Mix in the chili powder, cumin, cayenne, salt and pepper and cook 1 to 2 minutes more, then reserve on low heat. When cooked, remove the steak from the grill and set on a cutting board. Cool a few minutes and then thinly slice and arrange on a small platter. Spoon the onion/bell pepper mixture into a serving bowl and place it, the sliced steak, and the remaining ingredients, each in separate serving dishes, on the table and allow diners to create their own fajitas.
Instead of top sirloin, use any other tender beef steak in this recipe, such as strip loin or rib-eye. If you like things spicy, add a finely chopped jalapeno pepper to the skillet when cooking the onion and bell pepper. For added fibre, use whole wheat tortillas to make these fajitas.