¼ stick butter
½ cup chopped celery
½ cup coarsely chopped shallots
1 pound celeriac, peeled, woody parts removed, cut into ½ inch cubes
2 ½ cups chicken or vegetable broth
¾ tsp. fresh thyme
Melt butter in a heavy large pot over medium heat. Add celery; cover and cook until slightly softened, about 3 minutes. Add shallots; sauté uncovered about 3 minutes. Stir in celeriac and potato, then broth and thyme. Increase heat to high and bring to a boil. Reduce heat to medium-low, cover, and simmer until vegetables are tender, about 40 minutes. Cool slightly. Transfer soup to a blender and puree until smooth.