1. In a large saucepot, combine potatoes, celery, onion, garlic, and chicken stock. Bring to a boil, then lower heat to medium and simmer until all vegetables are very tender and potatoes break down, about 10 minutes.
2. Meanwhile, melt butter in a medium saucepot. When it starts to sizzle, stir in flour to make a roux. Cook the roux for a minute or two, stirring every few seconds so it doesn’t brown. Pour in milk all at once while stirring. Continue to heat over medium-low heat, stirring constantly with a wooden spoon or whisk, until milk mixture is thick and bubbly.
3. Using an immersion blender, carefully process the potato-vegetable mixture in the pot until mixture is smooth. Stir in rosemary.
4. Pour thickened milk mixture into potato mixture, stirring until completely incorporated. If mixture is very thick, add additional stock till desired consistency is reached. Taste for seasonings and add salt and pepper to adjust.

Recipe Notes:

• Although milk is called for in this recipe, you can use half-and-half if you want a creamier, richer texture. If you want to slim this down, use skim milk. Pureeing the potatoes give this soup a lot of body, so skim milk works fine.
• If you don’t have an immersion blender, let the potato-broth mixture cool slightly and process in batches in a blender or food processor.
• Add a smoky, salty note to this soup by replacing the butter with bacon drippings. Panfry 8 slices of bacon till crisp. Remove bacon to a plate lined with paper towels. Drain and discard all but 3 tablespoons of bacon drippings and add those 3 tablespoons to a saucepan. (If your bacon is very lean and you come up short on the drippings, just add a little butter or olive oil to make up the difference.) Proceed with the recipe from there. Crumble the cooked bacon and sprinkle over the soup immediately before serving.
• The amount of broth you will add depends on how starchy your potatoes are and how thin or thick you like your soup.
• Be sure to taste your soup before adding salt ~ some stock is fairly salty and you may not need to add additional salt.
• You can make this a vegetarian soup by replacing the chicken stock with vegetable stock. Make it vegan by using all vegetable stock and no milk and replacing the butter with margarine.