(Based on a recipe for poireaux mènagére from The Lutèce Cookbook )
4 large leeks (white part plus some green)
5 tablespoons butter, divided
1 large sweet onion, thinly sliced
12-14 medium Yukon gold potatoes
Cut leeks in half lengthwise, soak and wash thoroughly. Slice into 1/4-inch lengths. Quarter the onion and thinly slice.
Peel potaotes and cut into 1/4-inch slices; hold in water until ready to cook.
In a heavy-bottomed pot, melt 3 tablespoon butter. Add leeks and onions and, over medium heat, cook slowly until lightly browned. Drain potatoes and add to the pot. Pour in enough water to reach about 1/2 inch below the top of the vegetables; do not cover. Generously add salt and pepper, and stir.
Bring to a boil, cover and cook slowly for about 25-30 minutes. Vegetables should be soft, and some liquid will remain—do not discard liquid.
With a whisk, smash the vegetables in the liquid, breaking up the potatoes. Add the remaining butter, additional salt if necessary, and whisk until small chunks remain.