2 Tbs. extra-virgin olive oil
Kosher salt and freshly ground black pepper
4 1-inch-thick skinless mahi mahi fillets (about 6 oz. each)
1 small yellow onion, cut into small dice
1 medium clove garlic, minced
3/4 cup dry white wine
1 tsp. dried oregano
1 14.5-oz. can no-salt-added diced tomatoes
1/2 cup pitted green olives, rinsed, drained, and coarsely chopped
1 Tbs. capers, rinsed and drained
1/4 cup chopped fresh flat-leaf parsley
Heat 1 Tbs. of the oil in a 12-inch nonstick skillet over medium-high heat. Season the fish on both sides with 1/2 tsp. salt and 1/4 tsp. pepper. Cook, turning once, until the fish is just opaque throughout, 3 to 5 minutes per side. Transfer to a plate and keep warm.
Heat the remaining 1 Tbs. oil in the skillet over medium heat, add the onion and cook, stirring occasionally, until softened, about 3 minutes. Add the garlic and cook until fragrant, 30 seconds more. Stir in the wine and oregano. Bring to a boil, lower the heat to low, and simmer until the liquid is reduced by half, about 3 minutes.
Stir in the tomatoes and their juice, the olives, and capers. Bring to a simmer and cook until the sauce has reduced slightly, about 5 minutes. Stir in the parsley. Serve the fish topped with the sauce.