Heat a large frypan over medium-high heat. Add butter and oil and swirl pan until butter is melted. Add mushrooms, 3 or 4 sprigs of thyme, and garlic. Season with salt and pepper. Cook for 10 minutes, tossing often, or until mushrooms are golden brown and tender. Add spinach and pine nuts. Toss until the spinach is wilted. Remove from heat and set aside.
Cut approx. 10 cm off the fillet tail and trim the belly edge*. Cut fillet in half, so you have two pieces roughly the same size. Lay three pieces of sting across a baking tray lined with grease proof paper and place one half of fillet skin side down. Season and scatter remaining sprigs of thyme on top of the salmon. Place the mushroom filling on top, then cover with remaining salmon piece, skin side up. Tie kitchen twine around salmon to secure.
Roast in oven for 25 minutes – or BBQ on a hot plate for 8 to 10 minutes each side.
Transfer salmon to a platter, remove string and serve with crème fraîche.
Watch our video to see how to prepare the fillet: http://bit.ly/salmonroast