For the sauce: Heat the olive oil in a medium saucepan over medium-high heat. Add the onions and cook, stirring, until soft, about 3 minutes. Add the garlic and cook for 30 seconds. Add the tomatoes, thyme and 1/2 teaspoon of the salt, and cook until the tomatoes give up their liquid, about 5 minutes, stirring occasionally. Add the stock and tomato paste. Bring to a boil and cook until reduced by half, about 5 minutes. Add the cream and cook until reduced by half again, about 6 minutes.

Add the remaining 1/4 teaspoon salt and the butter. Remove the thyme sprigs. With an immersion blender, or food processor, puree the sauce. Strain sauce through a fine-meshed sieve, if desired, to remove the tomato skins. Keep the sauce warm until ready to serve.

For the salmon: Preheat the oven to 375 degrees F. Place dried mushrooms in a small food processor or spice grinder and process until finely ground; you will get about 5 tablespoons mushroom powder. Combine ground porcini, bread crumbs, shallots, garlic, thyme, and cheese in a small bowl; mix well. Drizzle in melted butter and stir to combine. Season with 1/4 teaspoon salt and 1/4 teaspoon freshly ground black pepper. Set aside.

Season the salmon fillets with the remaining salt and pepper. Heat the olive oil in a large, ovenproof skillet over medium-high heat. Add salmon fillets, skin-side up, to the pan and cook, about 2 minutes, or until golden. Carefully turn the fillets over in the pan and place a quarter of the porcini mixture over each fillet, gently pressing to adhere. Place the skillet in the oven and bake 5 to 6 minutes for medium. Serve hot with Tomato Cream Sauce