Instead of adding flour to thicken this soup, cooked tender peas take a whirl in the blender and do the job themselves. Buttermilk adds a gentle tang and dill really brings out the incredible flavor in this simple 30-minute soup recipe.
Combine broth, peas, spinach, onion, the chopped or dried dill (or savory), the salt, and pepper in a medium saucepan. Bring to boiling; reduce heat. Cover and simmer for 10 to 15 minutes for fresh peas (5 to 6 minutes for frozen peas) or until the peas are very tender. Cool mixture slightly.
Blend the pea mixture, half at a time, in a blender until smooth. Return the pureed mixture to the same saucepan. Stir in buttermilk; heat through. If desired, garnish with fresh dill sprigs.
To make ahead: Prepare as directed. Cover and chill the soup for up to 24 hours before serving.