Preheat oven to 180 ℃
Melt the butter in a medium saucepan over medium heat. Add the coconut flour and fry for 1 minute. Slowly add the stock, stirring the mixture the entire time so it stays smooth.
Next, slowly add the cream and stir until just simmering.Turn the heat to low and stir through the cheeses and mustard, along with a couple of grinds of salt and pepper.Cook for another 3 minutes. Place the fillets in a casserole dish small enough to fit the fish snugly.
Pour the sauce over the fillets.Bake for 30 minutes until the cheese is golden and bubbling. Let cool for 5 minutes before serving with a salad or mixed vegetables.