Heat oven to 350 F. Make sure all the liquid has been squeezed out of the spinach.
Heat a non-skillet skillet to medium high heat. Spray with olive oil or cooking spray. Add onions and cook until translucent. Add garlic and cook another minute. Add spinach, pesto, salt & pepper. Cook about 2 minutes, stirring to combine and remove from heat. Add ricotta and stir to combine once more.
Place salmon fillets in a baking dish (I used my Pampered Chef stoneware). Scoop spinach mixture evenly among the 4 salmon fillets. Bake for 15 minutes and serve.