Step 1

Heat 1 tablespoon oil in a large skillet over medium heat. Add onion; cook and stir until translucent, about 5 minutes. Add mushrooms and 1 teaspoon garlic; cook and stir until mushrooms are soft, about 5 minutes. Season with salt and black pepper. Transfer onion and mushroom mixture to a small bowl.

Step 2

Heat remaining 1 tablespoon oil in the same skillet. Add remaining 1 teaspoon garlic; cook and stir until fragrant, about 30 seconds. Add spinach; cook and stir until wilted, 3 to 5 minutes. Season with salt and black pepper. Transfer to another bowl.

Step 3

Preheat oven to 350 degrees F (175 degrees C). Line a glass baking dish with parchment paper.

Step 4

Dust work surface with flour. Lay 1 puff pastry sheet out on work surface. Spread onion and mushroom mixture evenly across pastry, leaving a 1/2-inch border. Center salmon on top. Sprinkle paprika over salmon and top with wilted spinach. Cover with second puff pastry sheet. Fold over and crimp edges to seal. Brush top with beaten egg. Transfer to baking dish.

Step 5

Bake in the preheated oven until pastry is golden, 40 to 45 minutes. Remove and let stand for 5 minutes. Slice into 4 portions.

Step 6

Whisk mayonnaise, lemon juice, Dijon mustard, and dill together in a small bowl. Spoon sauce onto 4 plates and place a piece of salmon on each.