Spinach Mushroom Filling
Place each salmon fillet, skin side down, on cutting board. At one end, cut between flesh and skin just enough to grip skin. Holding skin with paper towel and knife flat but without moving, pull skin back and forth to remove. Discard skin.
In small bowl, mash cream cheese to soften; spread over skinned side of fillets. Spread filling over 1 of the fillets, leaving ½-inch border. Top with remaining fillet, cheese side down.
With kitchen string, tie together at 1-inch intervals. Place on greased rimmed baking sheet. Spread butter over top; sprinkle with salt and pepper. (Make-ahead: Cover and refrigerate for up to 8 hours.)
Roast in 400°F oven until golden and fish flakes easily when tested, about 40 minutes. Transfer to cutting board; let stand for 5 minutes. Using serrated knife and sawing motion, cut crosswise into 1-inch thick portions.
Slice mushrooms. In large skillet, heat oil over medium heat; cook mushrooms, stirring occasionally, until golden and no liquid remains, about 10 minutes. Sprinkle with vinegar; toss to coat. Transfer to bowl.
Meanwhile, rinse spinach; shake off excess water. In large saucepan, cover and cook spinach over medium heat, with just the water clinging to leaves and stirring once, until wilted, about 3 minutes. Transfer to sieve; press out moisture. Chop and add to mushroom mixture along with salt and pepper. (Make-ahead: Cover and refrigerate for up to 24 hours.)