Set the oven to 425°F (220°C) and coat the inside of a baking dish lightly with grease. A thin film across the bottom helps keep the fish from sticking.
Blot the fillets with paper towels, combine the salmon’s lemon juice, olive oil, garlic, and Dijon in a small bowl, then arrange the fish in the dish, coat it with the rub, and season with salt and pepper. The surface should look evenly coated, with mustard grains and garlic distributed across each piece.
Roast the fillets for 10 to 12 minutes, checking that they have cooked through before removing them. The flesh should turn opaque and separate into flakes under gentle fork pressure.
Start the sauce during the bake by warming the butter in a small pot over medium heat until melted. The butter should be fluid without taking on a dark brown color.
Add the chicken stock to the melted butter, stir in the softened Boursin, and let the mixture simmer gently for a few minutes as it thickens. Small cheese pieces should gradually blend into a smooth sauce that lightly coats a spoon.
Move the pot off the burner before mixing in the sauce’s lemon juice. The finished sauce should remain creamy and flowing rather than bubbling hard.
Spoon the warm sauce across the cooked fillets while they remain in the baking dish. The cheese sauce will mingle with the juices collected around the salmon.
Scatter the chopped parsley over the fish if using. The green flecks add freshness against the pale sauce.
Bring the salmon to the table with the optional lemon slices. The fillets should sit in a pool of warm garlic-herb sauce.