In a large high-rimmed skillet (I use a cast iron skillet), melt the butter over medium-high heat. Once melted, add the onion and garlic. Fry, stirring frequently, until softened and translucent, about three minutes. Add the flour and stir well to coat the onion and garlic. Cook for about a minute, turning the heat down if the flour starts to color.
Whisk in the white wine and simmer until reduced by half. Add the fish stock and mustard and whisk until incorporated. Once the stock is at a gentle simmer, add the salmon fillets and leave to poach for 5 minutes. Turn the fillets over and cook for another five minutes. If your fillets are very thick, you may need to add a few more minutes to the cooking time. Using a lifter, transfer the cooked salmon fillets to a plate and keep in a warm place.
Slowly add the cream to the sauce while whisking constantly. Leave the sauce at a gentle simmer, stirring frequently, for about 5 to 10 minutes or until you have your desired consistency. Be sure not to let the sauce boil. Season with salt and/or pepper as desired.
I recommend serving the salmon fillets on a bed of rice and pouring over the sauce. Garnish with a nice green herb such as chervil or parsley. Enjoy!