Pat each salmon fillet dry and season generously with salt and black pepper. In a large skillet, heat a tablespoon of olive oil and a tablespoon of butter over medium heat until shimmering. Carefully place the salmon skin-side down in the skillet, searing for about 4–5 minutes until golden brown. Flip the fillets and cook for another 3–4 minutes until the salmon is cooked through and flakes easily with a fork.

Once the salmon is done, remove it from the skillet and set it aside on a plate. In the same skillet, add the minced garlic and sauté for about 30 seconds until fragrant. Then, stir in the fresh baby spinach, cooking for about 2 minutes until it's wilted and vibrant green.

Reduce the heat to low and pour in the heavy cream, scraping any browned bits from the bottom of the skillet for extra flavor. Add the grated Parmesan cheese, Dijon mustard (if using), and a pinch of red pepper flakes. Let the mixture simmer gently for 3–4 minutes until it thickens to a creamy consistency. Taste and season with additional salt, pepper, and a squeeze of fresh lemon juice.

Return the seared salmon to the skillet, spooning the creamy sauce over the fillets. Allow everything to simmer together for an additional 2–3 minutes, ensuring the salmon is heated through and well-coated. Serve immediately.