Preheat the oven to 390°F (200°C).
Sprinkle the fillets on both sides with salt and freshly ground black pepper.
In a saucepan, combine the lemon juice, butter, garlic, heavy cream, and Dijon mustard, and bring to a simmer. Season with salt and pepper. Simmer for 2 to 3 minutes, stirring until the butter melts and the sauce is fragrant, then remove from the heat and set aside.
Arrange the fish fillets in a single layer in a baking dish sprayed with cooking spray. Sprinkle the onion over the fish, then pour the cream sauce into the dish around and between the fillets.
Bake for 15 to 20 minutes, until the fish flakes easily with a fork.
Remove from the oven. Garnish with the lemon zest and fresh parsley, and serve with lemon wedges.