3 tablespoons butter
3 tablespoons flour
3 cups chopped leeks, white part only
4 cups potatoes, diced
1/3 to 1/2 cup heavy cream or sour cream,
and/or 2 to 3 tb butter
2 to 3 tablespoons minced parley and/or chives
Melt butter over medium heat. Add leeks, cooking without browning for 5 minutes. Stir in the flour and cook for 2 minutes. Remove from heat and gradually beat in a cup of hot water, blend well then add up to 5 more cups of hot water. Salt - about 1 tablespoon, pepper to taste, and add the potatoes. Simmer partially covered for about 40 minutes.
Mash or blend or process to obtain a smooth puree. Add as much cream or milk as you like, herbs and butter and stir to combine.
For the croutons I butter whatever stale bread I have and toast it for 10 minutes in a 400F oven.