Preheat the oven to 120C, fan-forced. Heat 2 tsp olive oil in a large frypan over medium-high heat. Cook the sausages, turning regularly, for 8 mins or until golden and cooked through. Transfer the sausages to an oven tray and keep warm in the oven. Reserve the pan.
Step 2 — Cook potato
While the sausages are cooking, peel the potatoes and cut into 3cm chunks. Put the potato in a medium saucepan of cold salted water and bring to the boil. Reduce the heat to medium and cook for 15 mins or until tender. Drain well and return to the pan. (For fluffy mash, start with dry potatoes. Once drained, leave in the hot saucepan for a few minutes to dry off.)
Step 3 — Caramelise onion
Meanwhile, thinly slice the onions. Melt 15g butter in the reserved pan over medium heat. Cook the onion, stirring, for 3 mins or until starting to soften. Add ½ tsp sugar and cook, stirring regularly, for a further 5 mins or until dark golden and caramelised. Bring a small saucepan of water to the boil for the greens.
Step 4 — Make mash and stock
Finely chop the parsley and mint leaves, discarding the stems. Add 2 tbs milk and 15g butter to the potato and mash until smooth. Taste, then season with salt. Stir in the mint and parsley and cover to keep warm. Crumble 1 stock cube* into a heatproof jug, add 1 tsp dijon mustard and 430ml (1¾ cups) boiling water and stir to dissolve.
Step 5 — Finish onion gravy
Scatter 1 tbs flour over the caramelised onion and cook, stirring constantly, for 1 min or until the flour starts to colour. Gradually add the stock, stirring constantly to prevent lumps forming. Bring to a simmer, reduce the heat to low and cook, stirring occasionally, for 5 mins or until slightly thickened.
Step 6 — Cook greens and serve
Cook the peas in the pan of boiling water for 2 mins or until bright green and tender. Add the spinach, stir to cover with water, then drain immediately. Divide the mash, greens and sausages among plates and pour over the onion gravy to serve.