Preheat oven to 425F. Line a shallow baking dish with parchment paper. Set in the fish in a single layer. Drizzle with the oil; season with salt & pepper. Bake 12-15min, or until just cooked through.
While the fish cooks, place the lemon juice in a small skillet, set over medium heat and reduce by half. Add whipping cream and reduce it by half. Add the dill, mussels and salt and paper to taste.
Cover and cook until the mussles open (2-3 mins). When cooked, divide the salmon among 4 plates; divide and arrange the mussels around the salmon and spoon over the sauce.
Garnish with dill sprigs and serve.
