Step 1: Pat each salmon fillet dry with paper towels. Season both sides generously with salt and freshly ground black pepper. Optionally, lightly dust with garlic powder. Let the seasoned salmon rest at room temperature for 10 minutes.
Step 2: Heat a large nonstick skillet over medium-high heat until shimmering. Add 1 tablespoon olive oil and 1 tablespoon unsalted butter, allowing the butter to foam. Gently place the seasoned salmon fillets skin-side down (if applicable), pressing lightly to maintain contact. Cook without moving for 4-5 minutes, or until the skin is deeply golden and crisp. Flip carefully and sear the flesh side for an additional 2-3 minutes until the center is just opaque (internal temperature 125-130°F for medium). Transfer to a plate and tent loosely with foil.
Step 3: Return the same skillet to medium heat. Add the sliced mushrooms in a single layer. Cook undisturbed for 2-3 minutes until golden underneath, then stir gently. Continue sautéing for another 3-4 minutes until the mushrooms release their liquid and develop deep, caramelized edges. Season lightly with salt and pepper. Transfer the mushrooms to a bowl.
Step 4: Lower the heat to medium. Add the remaining 1 tablespoon unsalted butter and the minced garlic to the emptied skillet, swirling for 30 seconds until fragrant. Add the fresh spinach and cook, stirring constantly, until just wilted (about 1-2 minutes).
Step 5: Pour in the heavy cream and low-sodium chicken broth, scraping any browned bits from the pan to incorporate flavor. Bring to a gentle simmer, then whisk in the fresh lemon juice and Dijon mustard until the sauce thickens slightly (after 2-3 minutes). Adjust seasoning with salt and pepper to taste.
Step 6: Return the salmon fillets to the skillet, nestling them gently into the creamy spinach sauce. Spoon the sauce over the tops of each fillet to coat evenly. Allow the salmon to reheat for 1-2 minutes, ensuring it's warmed through without overcooking.
Step 7: Sprinkle with chopped fresh parsley for color and brightness. Serve immediately with lemon wedges and your choice of side, such as cauliflower rice or zucchini noodles.