Grill the frozen sausages, following pack instructions.
While they're cooking, make the mash. Simmer the potatoes in water for 15-20 minutes until soft, then drain and mash with the milk.
To make the gravy, fry the red onions in the knob of butter over a medium heat for 5 minutes. Stir in the flour and 500ml stock, made using the beef stock cube, and simmer for a further 5 minutes.
Meanwhile, cook the frozen green beans in lightly boiling water for 3-4 minutes. Drain and serve immediately with the sausages, mash and onion gravy.