Preheat your oven to 190°C (375°F). Lightly grease a baking dish.
Cut a deep pocket lengthwise into each salmon fillet, being careful not to cut all the way through. Season with salt and pepper.
Heat butter in a skillet over medium heat. Add garlic and sauté until fragrant.
Add spinach and cook until wilted.
Stir in cream cheese, heavy cream, and Parmesan. Cook until smooth and creamy. Season to taste.
Spoon the spinach mixture evenly into each salmon pocket.
Place stuffed salmon in the baking dish and drizzle lightly with olive oil.
Bake for 20–25 minutes, or until salmon flakes easily with a fork.
While salmon bakes, melt butter in a saucepan. Whisk in flour and cook for 1 minute.
Gradually whisk in milk, cooking until thickened.
Stir in Parmesan, garlic powder, salt, and pepper until smooth.
Remove salmon from oven and spoon Parmesan sauce over the top before serving.