Preheat oven to 375°F. Line a rimmed baking sheet with parchment paper.
In a large sauté pan over medium heat, heat two turns olive oil and sauté mushrooms until browned. Season with salt and pepper, and cool.
For salmon, combine the cream cheese with herbs, lemon zest, garlic, salt and pepper. Halve salmon through the middle into 2 pieces of equal thickness. Season the salmon with salt and pepper.
Cut rolled dough into 2 pieces across the center, place 1 half sheet of dough on the baking sheet and center salmon on dough. Spread half of the cream cheese mixture on top of the salmon filet and top with other half of the salmon. Top with remaining cream cheese mixture and the cooled mushroom. Set pastry top in place and seal edges. Lightly score with cross hatch pattern on angle, being careful not to cut through dough. Bake 25 minutes, brush with egg wash and bake 20 minutes longer to cook salmon through and until the dough is puffed and brown.
For the salad, combine the crème fraiche with lemon juice, vinegar and EVOO to thin it out a bit. Season the dressing with sugar, salt and pepper to taste. Add the fennel and cucumber, radishes and onion, and toss to coat. Garnish with fronds and herbs of choice.
To serve, transfer salmon en croute to a serving board and slice. Serve hot or at room temperature with caperberries and salad alongside.