Pat the salmon fillets dry with a paper towel and season generously with salt and pepper on both sides.
Heat the olive oil in a large skillet over medium-high heat. Once hot, add the salmon fillets (skin-side down if using skin-on). Sear for 3-5 minutes per side until golden brown and cooked to your desired doneness. Remove salmon from the pan and set aside on a plate.
In the same skillet, melt the butter over medium heat. Add the sliced mushrooms and sauté for 4-5 minutes until they are browned and tender, scraping up any browned bits from the salmon.
Add the minced garlic to the pan and sauté for 1 minute until fragrant, being careful not to burn it.
Pour in the heavy cream and bring to a gentle simmer. Let it bubble for 2-3 minutes until the sauce slightly thickens.
Stir in the grated Parmesan cheese and lemon juice until the cheese is melted and the sauce is smooth.
Add the fresh spinach leaves to the sauce and stir gently until wilted, about 1-2 minutes.
Return the cooked salmon fillets to the skillet, spooning the sauce and vegetables over the top. Simmer for 1 more minute to reheat the salmon.
Garnish with fresh chopped parsley and serve immediately.