Preheat the oven to 375 degrees F. Generously grease a large rimmed baking sheet with olive oil.
Arrange the fillets on the baking sheet in one layer. Drizzle the 2 tablespoons of olive oil over the fish and season with salt and black pepper. Bake until the fish is flaky and just fully cooked through, about 12 minutes.
Meanwhile, finely grate the zest of 1 lemon and 1 orange into a small bowl and add the parsley, good olive oil, red pepper flakes, a pinch of salt, a few grinds of black pepper, juice of half the lemon and juice of half the orange. Stir gently to combine.
Slice the remaining lemon and orange. Serve the fish topped with the gremolata and citrus slices.