Preheat the oven to 325 degrees.
Place filets on a rimmed baking sheet.
Melt butter over low heat. Add diced onion and simmer until they soften. Off the heat add lemon juice and pour butter mixture over the filets.
Sprinkle the fish with paprika and bake in the preheated oven for 15 to 25 minutes, depending on the thickness of the fillets, or until the fish is cooked through and flakes easily with a fork.
Garnish with chopped parsley, lemon wedges, and serve!