In a bowl, whisk orange juice and zest, 1 tablespoon extra-virgin olive oil, vinegar, 1/2 teaspoon salt, mustard and 1/4 teaspoon pepper.
Gently stir in lentils, chopped mint and onion.
Arrange fillets on a foil-lined baking sheet. Brush with remaining 1 extra-virgin olive oil; season with remaining 1/4 teaspoon salt and 1/4 teaspoon pepper.
Roast until fish is opaque and just cooked through, 10 minutes.