Preheat oven to 400°. Generously season salmon all over with salt. Place on a plate and refrigerate until chilled, about 15 minutes.

Meanwhile, in a large skillet over medium heat, heat oil until shimmering. Add onion, garlic, and a generous pinch of salt and cook, stirring occasionally, until onion is translucent, about 5 minutes. Add spinach and stir until vibrant green and starting to wilt. Stir in cream cheese, Parmesan, and dill and cook, stirring frequently, until cheeses are melted and combined, about 2 minutes. Stir in ½ cup panko and cook, stirring, until mixture is thickened, about 2 minutes more; season with salt and pepper.

Rinse salmon under cold water to remove salt, then pat dry with paper towels. Place sheet of puff pastry on a cutting board and arrange salmon in the center. (There should be about 2" dough on each side of the salmon.) Season salmon with salt. Spoon spinach mixture over salmon, making sure it stays in the center of fish; smooth in an even layer. Sprinkle remaining ¼ cup panko over spinach mixture.

Fold long edges of dough over salmon, then fold short ends up. Flip seam side down and transfer to a parchment-lined baking sheet. Brush all over with egg wash, then cut shallow diagonal lines to score top of dough.

Bake salmon until pastry is golden brown and an instant-read thermometer inserted into thickest part of salmon registers 140°, 25 to 35 minutes. Transfer salmon to a platter and slice.