Make a nest of the lettuce in the bottom of the pan. Put the onion or mint (one or the other) the salt, butter and peas into the lettuce. Add the water, cover and gently boil until the peas are tender. Depending on the quality and freshness this may take anywhere from 5 to 20 minutes. Taste and season with extra salt, and perhaps, if they need it, a pinch of sugar. Serve with the lettuce and liquids in the pan.