Heat oil in a large skillet over medium heat. Pat the salmon fillets dry, then season flesh side with salt and pepper. Sear in the hot oil, skin side down, for ~7 minutes, then flip salmon fillets and sear the flesh side. While the flesh side is down, use a fork to peel the skin off of the top of the salmon. (Searing the skin first makes it easier to remove.) After ~4 minutes, flip again so that both sides have a chance to brown. Once cooked, remove from pan and set aside.
The pan should still have some juices in it from the salmon. Add butter to it, and warm over medium heat. Sauté the garlic and onion in the butter.
In a separate bowl, combine the half and half, the fat free half and half and the Greek yogurt. Stir until well combined.
Once the garlic and onions are cooked, reduce heat to low, and add the crème mixture to the pan, then add the mushrooms and Parmesan cheese. Stir constantly until simmering. Once simmering, add spinach leaves and stir into the sauce, allowing them to wilt and cook. Simmer for a few minutes, stirring occasionally, then add the salmon fillets back to the pan. Spoon the crème sauce over the salmon fillets, add fresh chopped parsley and thyme, and serve.
