Heat the butter and oil in a large, deep frying pan over a medium heat.
When the butter is sizzling, add the leek and a pinch of salt.
Cook, stirring occasionally, for 5 minutes until the leek is softened and just beginning to colour.
Stir in the spring greens and cook, stirring often, for 3 minutes until the greens are bright green, then pour in the stock.
Bring to a simmer and cook for 1½ minutes, then tip in the peas.
Season generously, then cover and cook for 3 minutes over a medium heat until the greens are tender and wilted and the peas are cooked through.
Stir through the vinegar and serve.
