Given the task definition and input, reply with output. In this task, you will be presented with the directions of a recipe separated by "," and have to fill in the "___" which is a step that is missing from the recipe.

1.,Saute celery and onions in oil until softened.,2.,Add vegetable broth and let simmer for about 25 minutes.,3.,______,4.,Select Variable 1.,5.,Turn machine on and slowly increase speed to Variable 10, then to High.,6.,Blend for 2 minutes.,7.,Serve immediately.
Place cooked ingredients, buttermilk, potatoes, garlic, salt, and pepper into the Vitamix container in the order listed and secure lid.