Coat a large piece of aluminum foil with butter.

Make a few cuts in the skin on each side of the fish. You don't need to cut all the way through, just enough to make room for the filling inside the fish.

Sprinkle salt and pepper inside the fish and stuff it with lemon slices and whole, rinsed parsley stems.

Wrap the foil around the fish and place it in a baking dish.

Bake the fish in the oven at 200 degrees Celsius (392 degrees Fahrenheit) for about 30 minutes.

Heat a pot with olive oil and sauté the shallots until they become translucent without browning.

Add white wine and let it reduce by half.

Add cream, and cook until the sauce reaches a creamy consistency.

Remove from heat and season the sauce with Dijon mustard, lemon juice, salt, and pepper.

Sauté shallots over low heat in a small pot with olive oil for a few minutes. Add white wine and let it simmer until only 1/3 of the wine is left in the pot. Pour in cream and cook until it becomes creamy in consistency. Remove from heat and whisk in some Dijon mustard. Season with salt, pepper, and a little lemon juice.

Serve the fish with steamed green asparagus, boiled rice, and the white wine sauce.