Preheat the oven to 200°C. Brown sausages in an ovenproof pan for 8-10 minutes, then set aside. Add butter, onions, thyme, bay leaf, and sugar to the pan. Stir, place the sausages back on top, and roast for 20 minutes, turning halfway.
Boil potatoes in salted water for 10-12 minutes until tender. Drain, heat the milk, and mash the potatoes into the milk. Mix in butter until smooth and fluffy.
Once the sausages are done, remove them. Caramelize onions, stir in flour, vinegar, red wine, soy sauce, and stock. Simmer until gravy thickens. Serve sausages, mash, and gravy together.