In a bowl, combine all the ingredients. Refrigerate to serve cold.
With the rack in the middle position, preheat the oven to 180 °C (350 °F).
In a skillet, melt half the butter. Add the spinach and cook until wilted, about 1 minute. Season with salt and pepper. Drain. Set aside.
With a sharp knife, butterfly the salmon fillet. To do this, slice it in half horizontally without separating it completely, then open it like a book. Put the fillet flat open on a parchment paper. Place the sole fillet right in the centre.
In a bowl, combine the remaining butter and lemon juice. Brush the sole fillet with this mixture. Season with salt and pepper. Cover with the spinach and shrimp. Arrange the asparagus in the centre of the fish.
Close by wrapping the salmon with the parchment paper. Wrap with aluminum foil and place in an ovenproof dish.
Bake for about 40 minutes or until the meat thermometer reads 58 °C (135 °F).
Let cool, then refrigerate overnight. Remove the paper. Slice and serve cold with the sauce.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.