Place fish in a 9" x 13" casserole dish then set aside.
Add onion, lime juice, garlic, chili powder, oil, salt and pepper into a blender or food processor then pulse until completely liquefied.
Pour mixture over fish then turn to coat all sides of fish in marinade. Allow fish to marinate for at least 45 minutes. Meanwhile, preheat indoor or outdoor grill to medium heat.
Remove fish from marinade. Grill for 3-4 minutes per side, depending on thickness. Cook fish just until nicely browned and flakes easily when pierced with a fork.
Remove from grill then break into pieces. Add desired amount of fish to tortilla then fill with your favorite accompaniments.
Notes
The prep and cook time are next to nothing, all the heavy lifting takes place while the fish marinates in our yummy sauce!