Preheat oven to 200°C / 390°F (all oven types).
Place fish in a baking dish – ensure the fish isn’t crammed in too snugly.
Sprinkle both sides of fish with salt and pepper.
Place butter, cream, garlic, mustard, lemon juice, salt and pepper in a microwave proof jug or bowl. Microwave in 2 x 30 sec bursts, stirring in between, until melted and smooth.
Add xanthan gum and blend with immersion blender
Sprinkle fish with shallots, then pour over sauce.
Bake for 10 – 12 minutes, or until fish is just cooked. Remove from oven and transfer fish to serving plates. Spoon over sauce, and garnish with parsley and lemon wedges if using.