Preheat the oven to 400 degrees F (200 degrees C). Line a 9x11-inch baking pan with foil.
Press as much liquid from the thawed spinach as possible using clean dish towels or paper towels and place in a medium mixing bowl.
Melt butter in a small, nonstick skillet over medium heat. Stir in mushrooms and onions and cook, stirring occasionally, until the onions begin to turn clear, about 4 minutes.
Add cooked mushrooms and onions to spinach with Parmesan cheese, garden vegetable cream cheese, and garlic powder. Stir until well combined and add kosher salt and pepper. Set aside.
Combine olive oil and lemon juice in a small bowl.
Pat salmon fillets dry and brush both sides with the combined olive oil and lemon juice. Place fillets in the foil-lined pan and sprinkle with ground sea salt. Divide spinach mixture evenly between the two fillets.
Bake in the preheated oven until the salmon flakes easily with a fork, 15 to 19 minutes. Garnish with lemon slices and fresh parsley.
