Serves 4-6.
1 Tbsp olive oil
2 Tbsp butter
1 medium onion, diced
3 medium red-skinned potatoes, diced (do not peel)
1 large Idaho baking potato, peeled and diced
2 Tbsp roughly chopped fresh dill weed, plus more for garnish
1/2 tsp dried thyme or 1 tsp fresh thyme leaf
4 cups homemade or low-sodium chicken stock
Plenty of kosher salt and fresh black pepper to taste (if using store bought stock, you probably will not need extra salt here)
Greek yogurt or heavy cream (optional)
In a Dutch oven (I use a 4-quart size), heat the olive oil and butter. Cook the leeks and onion in the pot over low heat until limp but not brown. Add remaining ingredients except yogurt or cream, and bring to a boil. Cook, covered, over low heat until potatoes are tender, 20 minutes or more.
Puree the soup with an immersion blender, or in a food processor (in batches, if necessary) and return to the pot. Taste, and adjust seasoning with more dill, salt and pepper, if necessary. Add more stock or a bit of water if the soup is too thick (usually not needed).
If you're going to serve this cold, remember that you will need a bit of extra seasoning and salt.