Prepared in 50 minutes or less.
Construct bouquet garni inside cheesecloth. Close and tie with kitchen twine. Set aside.
In a large Dutch oven on medium heat, melt the butter and warm olive oil. Add leeks, minced shallots and garlic; cook until very soft approximately 10 minutes occasionally stirring.
Do not brown. Add cubed potatoes, chicken stock and bouquet garni. Bring to a boil before reducing heat to gentle simmer. Cook until potatoes are fork (very) tender, approximately 40 minutes.
Discard bouquet garni. Working in batches, pass soup mixture through a food mill with a medium disk or purée in a blender leaving some texture. Warm soup over medium-low heat.
Slowly stir in whole milk and cream then season with salt and fresh white pepper. Once milk is added, do not boil soup.
Spoon into small bowls and garnish with chervil leaves of chopped chives. Serves 4.