Heat olive oil in a skillet over medium heat and add salmon fillets, skin-side down.
Cook for 4–5 minutes on each side until golden and just cooked through, then remove and set aside.
In the same skillet, melt butter and sauté minced garlic for about a minute until fragrant—don’t let it burn.
Add mushrooms and cook until softened, about 3–4 minutes, then stir in spinach and let it wilt gently.
Pour in heavy cream, stirring to combine and scraping up any bits from the pan.
Simmer for 2–3 minutes until slightly thickened, adding a splash of milk or broth if needed.
Return salmon to the skillet, spoon the creamy sauce over the top, and let it all simmer for another 2 minutes.
Season with salt and pepper to taste, and serve warm.