"White fish fillets and shrimp are marinated with achiote, orange juice, lemon, and spices, baked in the oven, and served with habanero sauce and red onion. You can use any white fish fillets. These are typical flavors of Southeast Mexico."
Combine 3/4 cup water, achiote paste, onion, juice of 2 limes, juice of 1 orange, vinegar, garlic, 1 teaspoon oregano, salt, and pepper in a blender; blend until smooth.
Season both sides of the fish fillets with salt and pepper, then brush the top with melted butter. Arrange fish fillets in a baking dish. Add shrimp on top and cover with achiote sauce. Cover baking dish with aluminum foil.
Bake in the preheated oven until fish flakes easily with a fork, about 30 minutes.
Place red onion and habanero peppers in a glass bowl. Add 1 cup water, juice of 1/2 orange, and juice of 1 lime; stir well and season with salt, peppercorns, and Mexican oregano.
Serve fish and shrimp with achiote sauce and top with habanero sauce.