I could not find a recipe for this on the internet, so I put few together.You can assemble it ahead of time, bake it ahead of time, doesn't matter: your guests will leave no leftovers.
2 packages 20 oz. shredded hash browns (or equivalent in shredded potatoes, knock yourself out).
1/2 cup margarine (or butter)
2 Tablespoons olive oil (or other oil)
1 Tablespoon flour (I used rice flour. Potato starch, regular flour, matza meal, all would work)
1 can condensed mushroom soup (parve, kosher for passover but can substitute).
Salt and Pepper
Bread crumbs optional
Preheat oven to 375F
Melt 1/4 cup of margarine in a small sauce pan, add the flour. Cook until it thickens. Remove from heat and add the mushroom soup. If it is condensed, you can add water. Add heat, and bring to a slow boil with stirring. It should be thick, and coat the back of your spoon.
Chop the leeks and fry them in a little oil and margarine until they are soft. Put them in a bowl, and fry the hash browns following the package instructions (or how you like) with the rest of the oil and margarine. I fried the potatoes in 2 batches, one package of hash browns at a time. Break up the hash browns, add to the leeks, and smooth over the bottom of a 9 x 12 casserole pan. Add the mushroom soup sauce. Salt and pepper to taste. I tried with and without bread crumbs, and liked it better without.
Bake at 375F for 35 minutes.