Bangers ‘n’ Champ with Red Onion Gravy

Name: Bangers n Champ with onion gravy
Prep time: 10 min
Cook time: 30 min
Total time: 40 min
Ingredients: 8 sausages; 2 red onion (sliced); 2 tbsp gravy granules; 1 sprig thyme; 1 kg white potatoes (peeled, cut into chunks); 3 knob butter; 60 ml milk; 2 spring onions (finely sliced); 500 g peas; 2 tbsp English mustard (to serve); 1 pinch sugar