Peel and chop the potatoes into chunks.
Boil the potatoes in salted water for about 15 minutes until tender.
While the potatoes boil, heat olive oil in a pan over medium heat.
Add sausages and cook until browned on all sides, about 10-12 minutes.
Remove sausages and set aside, keeping them warm.
In the same pan, add sliced onions and sauté until soft.
Stir in flour and cook for 1 minute to form a roux.
Gradually add stock, whisking to prevent lumps, and simmer until thickened.
Drain potatoes and mash with butter and milk until smooth.
Serve sausages over mash, topped with onion gravy.