Gather and prep all ingredients. Pre-heat oven to 400F and grease a baking sheet or casserole dish that will fit the salmon.

Season the skin-side of the salmon with salt and pepper, then lay it on the baking sheet or in the casserole dish. Season the top with salt and pepper as well.

Mix together the topping by combining pesto, mayonnaise, and parmesan cheese in a small bowl.

Spread the pesto mixture over the top of the salmon fillet(s) making sure to distribute evenly. Place in the oven and bake on the middle rack for 20-30 minutes or until salmon flakes with a fork when pulled apart.

About 5-8 minutes before the salmon is done cooking, heat olive oil in a pan over medium heat. Once hot, add spinach and cook until soft and tender, about 3-5 minutes. Season with salt and pepper to taste.

Plate up the salmon and spinach, then sprinkle crumbled walnuts over the top of the dish. Enjoy!