Heat the oil in a large skillet over medium-high heat. Season the fish fillets on both sides with salt and pepper, then sear in the hot pan, flesh-side down first, for 5 minutes on each side so they seal. Once cooked, remove fish from the pan and set aside. Melt the butter in the remaining juices leftover in the pan. Add in the garlic and fry until fragrant, then fry the onion in the butter mix.Add the cream, mustard and lemon juice, then cook until melted and smooth. Add in the white wine. Bring to a gentle simmer, while stirring occasionally. Season with salt and pepper to your taste. Next, add in the spinach leaves and allow to wilt in the sauce, then leave sauce to simmer for a further minute until ready to serve. Add the salmon back into the pan; sprinkle with the parsley, and spoon the sauce over each fillet, before presenting in the skillet.