Add butter and olive oil to a large soup pot set over medium heat. Once the butter has melted, add diced onion and sprinkle over top ½ teaspoon kosher salt. Saute for 3-4 minutes, or until the onion is softening and becoming golden.
Add the celery and potatoes and season with remaining 1 teaspoon kosher salt. Saute for 8-9 minutes, then add minced garlic. Saute for 1 additional minute.
Pour in a little bit of the chicken broth to deglaze the pan and scrape up any bits stuck to the bottom. Add the rest of the chicken broth, the water, and the bay leaf and raise heat to bring everything to a boil.
Once boiling, reduce heat to medium-low and cook for 20 minutes, or until the potatoes have softened.
Once the potatoes have softened, remove the bay leaf and take the pot off the heat. Ladle out some of the liquid and add it to a bowl. Set this aside for now.
Puree the soup with an immersion blender, or very carefully transfer the hot soup to a blender to puree until smooth. This may need to be done in batches as to ensure there's enough room to prevent the hot soup from spraying out.
Once pureed, set the soup back on the stovetop over low heat. Now add yogurt to the small bowl of portioned-out broth and whisk well, until smooth and combined.
Whisk the yogurt/broth mixture into the saucepan of soup. Stir in rosemary, rubbed sage, pepper, and/or whatever herbs and seasonings you like, then serve.
Ingredient NotesPotatoes: Yellow/Yukon gold potatoes have a slight edge over Russet potatoes as they end up a little creamier when blended in a soup, though Russets are perfectly fine to use here, too.Greek Yogurt: Whole milk/Full-fat (5% or so) is recommended for a little boost in this healthy creamy soup, though lower fat varieties (2%) can also work. I don't recommend nonfat.Sage: Finely diced fresh sage can be used in place of the rubbed sage if that is what you have on hand. I'd start with 3/4 teaspoon of diced fresh sage and see if you'd like to add more. Ground sage is more concentrated in flavor than both fresh and rubbed sage so you would need less. Start with 1/8 teaspoon of ground sage if using.Recipe Tips
If intending to freeze this soup, consider not adding the yogurt before doing so to prevent any texture weirdness/grainy separation. Then whisk the yogurt with some chicken broth and add it to the soup when reheating.
Please don't overfill your stand blender if blending the soup that way. It'll be very hot and will splatter up when it starts. Leave sufficient empty space and blend the soup in batches.