Preheat the oven to 200°C.
Roast sausages in a tin for 10 minutes, turn, and roast for 5 minutes more.
Add sliced onions and a mix of herbs, mustard, and stock to the tin.
Bake for 20 minutes until the sausages are cooked and the liquid thickens.
Boil potato cubes in water until tender then drain and mash.
Heat 50g butter and 100ml milk then stir into the mashed potatoes until smooth.
Boil peas in a saucepan for 3 minutes and drain thoroughly.
Create a paste from 2 tsp softened butter and 2 tsp flour.
Remove sausages from the tin and whisk the flour paste into the onion gravy on the hob.
Return sausages to the thickened gravy and serve over the mash and peas.