Pat salmon fillets dry with paper towels and season both sides with salt and pepper.
Heat olive oil in a large skillet over medium-high heat.
Place salmon in the skillet skin-side up and sear for 5 minutes, then flip and sear for another 3-4 minutes until cooked through; remove from pan and set aside.
In the same skillet, melt butter over medium heat.
Add mushrooms and sauté for 5-7 minutes until browned and moisture has evaporated.
Stir in minced garlic and cook for 1 minute until fragrant.
Add baby spinach and cook until just wilted.
Pour in heavy cream and bring to a gentle simmer.
Stir in parmesan cheese and lemon juice, simmering for 2-3 minutes until the sauce thickens.
Return the salmon fillets to the skillet, spooning the sauce over them.
Garnish with fresh parsley and serve immediately.