Leek Potato Soup

This is one of our favorites!  If I don't have leeks I sometimes just alter the recipe and add onions, though it is the absolute best when you follow the ingredients needed (even if you aren't exact).  Also to make a little more sometimes I throw in a few extra potatoes and another cup or 2 of broth.

1 lb leeks, cleaned and dark green sections removed (abt 4-5 med. leeks), and then sliced
3 tablespoons butter
heavy pinch salt
5-7 potatoes, peeled and diced
4 cups vegetable or chicken broth
1 cup heavy cream
1 cup buttermilk
1/2 tsp pepper
1-2 teaspoons herbs de Provence (or if you don't have that, a mixture of rosemary and thyme works too)

In a 6-quart pan/pot, on medium heat, melt the butter and add the leeks and a heavy pinch of salt and sweat for 5 minutes.  Decrease the heat to medium-low and cook until leeks are tender, stirring occasionally.  Add the potatoes, broth, and herbs and increase heat to med-high, and bring to boil.  Reduce the heat to low, cover and simmer until potatoes are soft, approximately 45 minutes.
Turn off heat and puree mixture with an immersion blender until smooth.  Stir in the heavy cream, buttermilk, and season with extra salt to taste.