Step 1

Preheat the oven to 400 degrees F (200 degrees C). Line a large baking sheet with parchment paper.

Step 2

Stir olive oil, lemon juice, chili powder, cumin, onion powder, garlic salt, black pepper, and cayenne together in a bowl.

Step 3

Spread bell peppers and onion on the prepared sheet pan. Pour 1/2 the marinade over vegetables; toss until well coated.

Step 4

Roast vegetables in the preheated oven for 10 minutes.

Step 5

While vegetables are cooking, place mahi mahi in a bowl and cover with remaining marinade; toss until well coated. Let sit for 10 minutes.

Step 6

Remove vegetables from the oven and set mahi mahi pieces on top. Return to the oven and cook until fish is cooked through and flakes easily with a fork, about 12 more minutes.