Add the grapefruit and lemon juice, and seasonings, to a large zipperlock freezer bag, or a large container with a lid. Seal, and shake until the ingredients are blended.
Add the beef, and refrigerate from 4 to 12 hours.
When the beef is ready, preheat your grill (indoor or outdoor) to a high setting.
Slice the onion into thin wedges.
Remove the stems and seeds from the pepper, and cut into thin strips. Add to the onion slices in a bowl along with the tablespoon of olive oil. Toss to coat the vegetables.
Spread the veggies in a single layer on a grill pan, or a large piece of foil placed on the grill. Grill for about 10 minutes, until the onions are browned and the peppers are softened.
Keep the veggies warm as you sear the steak for about 5 minutes on each side.
Let the meat rest a few minutes before cutting into strips crosswise.
Serve with vegetables and toppings in the tortillas.