In a medium bowl, combine diced mango, red onion, jalapeño, and cilantro. Squeeze lime juice over top, add sea salt, and toss gently. Set aside for at least 15 minutes.
Pat the mahi mahi fillets completely dry with paper towels. In a small bowl, mix chili powder, cumin, garlic powder, kosher salt, and black pepper. Season all sides of the fish fillets.
Heat olive oil in a large skillet over medium-high heat until it shimmers.
Carefully place the mahi mahi fillets in the hot pan. Sear for 3-4 minutes per side, undisturbed, until a golden-brown crust forms.
The fish is done when it is opaque and flakes easily with a fork (internal temperature of 137°F).
Remove fish from the pan and let it rest for 2 minutes. Serve immediately, topped with a generous amount of the mango salsa.
Notes
Ensure the mahi mahi fillets are completely dry before seasoning. This is the secret to getting a perfect, golden-brown sear without the fish steaming in the pan.