Pat the salmon very dry on both sides. Season with salt and pepper. Heat skillet over medium-high heat with just 2 tablespoons of olive oil. When hot, place salmon skin side down. Cook for 2 minutes, until the skin is crispy.
Turn the heat down to medium, flip the salmon. With a piece of tin foil, cover the salmon loosely. This will cook the salmon while still allowing the steam to escape, keeping the skin crispy. Cook for 3-4 minutes, until the salmon is nearly cooked through, pink in the middle (we will finish cooking the salmon at a later step.) Remove the salmon to a clean plate.
Return the skillet to the stove and turn the heat to medium-high. When hot, swirl in the remaining 1 tablespoon of olive oil. Add in the onions and the mushrooms. Cook for 3 minutes, until the onions are softened and the mushrooms are nicely browned.
Add in the minced garlic and the spinach. Stir and let the garlic sizzle in the oil for a few seconds until fragrant. Add in the white wine and let cook for 1 minute until alcohol has evaporated. Stir in heavy cream. Season with salt and pepper, to taste.
Return the cooked salmon to the pan, and let it snuggle in the creamy sauce - let the salmon finish cooking, about 30 seconds to 1 minute. The spinach should be wilted but still bright green.