Line a large baking sheet with parchment paper or a silicone liner. In a small bowl, combine chili powder, paprika, cayenne pepper, salt, and black pepper. Sprinkle the seasoning mixture evenly over both sides of the fish.

Drizzle the fish lightly with olive oil and dot with butter. Bake at 375ºF (190ºC) for 20–25 minutes. Broil for 3–5 minutes at the end if desired to brown the edges. Let the fish rest for 5 minutes, then flake gently with a fork.

In a bowl, combine the chopped red onion and tomatoes with a few drops of lime juice to create the salsa.

In another small bowl, mix mayonnaise, sriracha, garlic powder, and remaining lime juice to make the spicy sauce. Adjust to taste.

Toast the corn tortillas in a dry skillet over medium-high heat for about 30 seconds per side, or warm them between plates in the microwave for about one minute.

Assemble by placing flaked fish onto each tortilla, topping with salsa, and finishing with a drizzle of spicy sauce. Serve with lime wedges.