Heat oil in a deep frying pan and sauté the red onion and pheasant for 3 minutes until colored.
Add red wine, balsamic vinegar, stock cube, and butter to the pan.
Simmer the mixture for 15 minutes until the sauce is reduced and pheasant is cooked through.
Preheat the grill to the highest heat setting.
Boil the diced potatoes in a saucepan of water for 5 minutes.
Drain the potatoes and mix in the mustard and rosemary using a fork.
Press the potato mixture into a ring mould centered on a baking sheet.
Top the potatoes with Stilton and grill for 3 minutes until bubbly.
Serve the pheasant alongside the potatoes and drizzle with the red wine sauce.