In a large pan, heat the oil over medium heat.
Add the mushrooms and cook, stirring occasionally, for about 10 minutes or until they are golden brown and the mushroom liquid has evaporated.
Sprinkle with balsamic vinegar and mix well to coat.

Add the spinach and cook for 3 minutes, stirring regularly.
Add half of the salt and pepper and mix.

Place the salmon trout fillets, skin side down, on a cutting board.
Insert the blade of a sharp knife between the flesh and the skin, just enough to hold the skin. Holding the skin with a paper towel and keeping the knife blade flat without moving it, pull the skin off by making a forward and backward motion. Remove the skin.
Repeat these operations with the other salmon trout fillet.

Spread the Boursin on each of the salmon trout fillets.
Cover one of the salmon trout fillets with the mushroom filling, leaving a border of about 1/2″ (1 cm) around the edge.
Cover with the other salmon trout fillet, the cheese underneath.

Using a string, tie the salmon trout fillets, leaving a space of about 1″ (2.5 cm) between each turn of the string.
Place on a baking sheet with edges, greased.
Spread the butter on top of the stuffed salmon trout fillet. Sprinkle with the remaining salt and pepper.

Bake in a preheated oven at 400 F (200 C) for about 40 minutes or until the salmon is golden brown and flakes easily with a fork.
Place on a cutting board and let rest for 5 minutes.
Using a serrated knife, carefully cut the salmon trout fillet into slices of about 1″ (2.5 cm) thick.