Normally, when planning to use butter to flavor or top a dish, I make a compound butter and incorporate flavors into it before adding it to my dish. In the case of this particular dish, because everything steams together in a foil packet (papillote if you want to be fancy), I just bung everything onto the top of the fish and let the flavors meld as they cook. This is another quick, simple and delicious fish recipe for a quick weeknight supper.
4 flounder fillets
1 orange
1 lemon
4 tsps butter
1 tsp caper or pickle brine or 1/2 tsp Yuzu-Kosho or 1 tsp rice wine vinegar
Salt
Preheat oven to 375.
Place each flounder fillet on the top of an individual piece of foil, large enough that it can be folded into a packet around the fish. If using Yuzu-Kosho, spread 1/4 on each of the fillets at this point in time.
Cut the butter into small dice and dot a 1/4 onto each fillet. Grate some of the orange zest over each of the fillets and add a generous squeeze of the juice from the orange. If using brine or vinegar, sprinkle 1/4 over each piece of fish. Season lightly with salt.
Fold the aluminum around the fish into packages (with the ends pointing up to avoid oven leakage). Place in oven and bake until done. This will depend on the thickness of the fillets; my very thin ones took exactly 10 minutes.
Remove from oven. Open each papillote, taste a tiny bit of the accumulated juices and squeeze a bit of lemon juice over the top accordingly.
When making this as I did today, with herbed couscous, I pour the juices from the fish into the couscous, fluff it up and serve it and the fish nicely on a serving dish with no foil in sight. You can also serve the fish directly in the papillote at table, along with crusty bread for sauce retrieval purposes.