In a medium skillet, set over medium-low heat, add the butter. When melted, add the leeks, a liberal pinch of salt and a few rounds of black pepper. Cook until the leeks are soft, stirring regularly, about 10 minutes. Add the soy sauce, minced garlic, and grated ginger and cook for 1 minute more, until the garlic is fragrant. Add the reserved mashed peas and whole peas. Give it a good mix and transfer to a bowl. We’re not really cooking the peas a whole lot—we want that fresh flavor! Give it a taste and adjust the salt, if you like.