Melt the butter in a large pan and fry the red onion until softened.
Crush the juniper with a mortar and pestle and add to the onions.
When the onions are browning, stir in the remaining ingredients. With a fork, mash the potatoes and pumpkin, distributing it evenly to bind the meat.
Add the Brussels sprouts and cook through for 15 minutes, adding more butter if needed. (If you only have leftover cooked Brussels, you can add these, but cook for about five minutes.)
