Trim the salad onions and, if the bulbs are very large, cut them in half. Cut the fleshy bits of the green parts into short lengths.If using shallots, peel, trim and, unless they’re very small, cut in half.
Spread half the butter over the base of a medium saucepan, then use the outer leaves of the lettuce to line the pan.
Add the peas, onions, half the mint, a good pinch of salt, the remaining butter, diced and spread out among the peas, and the wine.Top with the rest of the lettuce, so the peas are encased.
Put the pan over a medium heat and bring to a simmer.Cover, turn down the heat as low as possible and leave to cook very gently for about 30 minutes, shaking the pan occasionally, until the peas are soft and buttery.
Scoop out the vegetables into a warm serving dish with a slotted spoon and taste – if you think the sugar is required, add it to the liquid in the pan.
Keeping the veg warm, bring the liquid in the pan to a simmer and leave it to bubble away until it’s thickened slightly. Pour over the pea and lettuce mix, scatter over the remaining mint and serve.