Heat olive oil in a large skillet over medium-high heat. Season the salmon fillets with salt and pepper.
Once the oil is hot, add the salmon fillets skin-side down. Cook for about 4-5 minutes until the skin is crispy. Carefully flip the fillets and cook for another 3-4 minutes until cooked through. Remove the salmon from the skillet and set aside.
In the same skillet, add 2 tablespoons of butter. Once melted, add the minced garlic and sliced mushrooms. Sauté for about 5 minutes until the mushrooms are tender.
Add the fresh spinach to the skillet and cook until wilted, about 2 minutes.
Pour in the heavy cream and lemon juice, stirring to combine. Let the sauce simmer for 2-3 minutes until slightly thickened. Season with salt and pepper to taste.
Return the salmon fillets to the skillet, spooning the creamy sauce over them. Cook for an additional 1-2 minutes to heat through.
Remove from heat and garnish with chopped parsley before serving.