Preheat oven to 400 degrees F.
Line a rimmed baking sheet with aluminum foil. Lay the lemon slices on the baking sheet so the Mahi Mahi can sit on top of them.
Place the fish on top of the lemons and brush them with the butter on both sides.
Combine the paprika, garlic powder, oregano, salt, thyme, red pepper flakes, onion powder, and black pepper in a small bowl. Stir and sprinkle over the fish, coating both sides with seasoning.
Bake the fish for 10 minutes, then broil on high for 1 ½ minutes.
Remove from the oven and top with parsley. Serve immediately.