Bring the salmon to room temperature. Preheat an oven to 375°F (190°C), then line a rimmed baking sheet with parchment paper. Allow the salmon to sit on the counter while you make the filling.
Cook the spinach. Heat 1 Tbsp olive oil in a skillet over medium heat. When hot, add fresh spinach, then cook for 1-2 minutes, stirring often. When the spinach is almost wilted, add grated garlic and a generous pinch of kosher salt. Cook another minute, or until the spinach is wilted. Remove from the heat and set aside.
Make the filling. Combine 4oz cream cheese, ¼ cup mayo, ½ cup shredded Gruyere, 2 Tbsp grated Parmesan, lemon zest, ½ tsp kosher salt, and ¼ tsp black pepper in a mixing bowl. Mix until well combined. Add chopped artichoke hearts and the cooked spinach, then stir together.
Prepare the salmon. Pat the salmon filets dry with a paper towel. Working on a cutting board, use a paring knife to cut a long slit down the middle (top) of each filet, being careful not to cut all the way through. You should leave about ½-inch on either side of the cut. Drizzle each filet with a little olive oil. In a small mixing bowl, combine the garlic powder, onion powder, paprika, Italian seasoning, kosher salt, cayenne, and black pepper. Sprinkle evenly on top of the salmon.
Stuff the salmon. Divide the mixture evenly between salmon filets, then gently press the spinach mixture into each salmon pocket. Place each filet on the prepared baking sheet.
Bake. Transfer to the oven, then cook for 15 minutes, or until the salmon reaches an internal temperature of 120°F (49°C). Note that wild salmon (thinner) should be checked around 12 minutes, while thicker cuts of salmon may take a few minutes more.
Serve with lemon wedges and a pinch of flaky sea salt. Enjoy!