Step 1 Spray a large saucepan with oil and place over medium-low heat. Add onion and cook, stirring occasionally, for 10 minutes, until slightly caramelised. Add capsicum. Cook, stirring occasionally, for 5 minutes. Add vinegar and stir to combine. Cook for a further 2–3 minutes, until caramelised.
Step 2 Meanwhile, steam potatoes for 10 minutes, adding peas in the last 3 minutes of steaming. Return to saucepan, add milk and mash until smooth. Set aside and keep warm.
Step 3 Spray a large frying pan with oil and place over medium-high heat. Cook sausages for 5 minutes, turning, or until cooked through. Divide mash and sausages between serving plates and top with balsamic onions and capsicum.