Preheat the oven to 200C/400F/Gas 6.
Place the sausages into a roasting tin and place in the preheated oven. Cook for 10 minutes then turn the sausages and cook for a further five minutes, or until they have a good colour on the outside.
Add the sliced onions to the tin. Mix the dried herbs, mustard and stock together and pour over the sausages and onions. Return the tin to the oven for a further 20 minutes, or until the sausages are cooked through and the onion gravy thickened. Season to taste with salt and pepper.
For the mash, boil the potatoes in a pan of boiling water until tender. Drain and mash.
Heat the butter and milk until the butter has melted. Add a pinch of salt and pepper then pour over the mashed potato and mix until smooth. Set aside until ready to serve.
Cook the peas in a saucepan of boiling water for 2-3 minutes, or steam them for 1-2 minutes. Drain thoroughly and set aside.
For the sausages and onion gravy, mix the softened butter and plain flour together to form a paste. Remove the sausages from the baking tray, set aside and cover to keep warm. Place the baking tray onto the hob, add the flour and butter mix and whisk until combined. Stir over a medium-high heat for 2-3 minutes, or until the gravy has thickened slightly. Add the sausages back to the tray and warm through for 1-2 minutes.
Serve the sausages, mash and peas with the onion gravy spooned over.