Melt the butter in a large, deep saucepan with a tight-fitting lid over medium heat. Add the onion and cook for 10–12 minutes until starting to brown. Add the sherry and leave to bubble for 3 minutes. Add the mussels and stir with the onions and liquid. Cover and steam for 10 minutes, stirring once.
Open the pot, add the crème fraîche and toss again. Place the kale over the mussels and