Preheat the oven to 400°F (200°C). Grease a baking dish with half of the butter or oil.

Salt and pepper the salmon fillets and place them in the prepared baking dish, skin-side down. Mix mayonnaise, pesto and parmesan cheese and spread over the salmon. Bake for 15–20 minutes, or until the salmon is done and flakes easily with a fork. Meanwhile, sauté the spinach in remaining butter or oil until it’s wilted, about 2 minutes.

Season with salt and pepper. Serve immediately with the oven-baked salmon.