Heat the oil in a large saucepan over a medium heat and fry the onions for 10-15 mins until soft and lightly golden, stirring frequently. Add a splash of water if the onions are taking on too much colour or sticking. Stir through the flour until incorporated, then pour in the stock. Simmer for 10 mins until thickened. Stir in the vinegar and sugar.
Meanwhile, preheat the grill to medium-high. Put the sausages on a baking tray and grill for 15-18 mins, turning occasionally, until golden and cooked through.
Boil the potatoes for 10-15 mins until a sharp knife pierces them easily. Add the greens and peas to the pan, bring back to the boil, then simmer for 2-3 mins more until the veg is tender and bright green. Drain well, then tip back into the warm pan. Mash briefly to crush the potatoes and stir together; season well. Pour the gravy into a jug and serve alongside the spring green mash and sausages.
