Look for kohlrabi, a mildly sweet purple or green vegetable in the cabbage family, at better supermarkets. For a creamier, smoother purée, pass the boiled ingredients twice through a potato ricer or food mill.
Ingredients
2 pounds celery root (celeriac), peeled, cut into 3/4" cubes
Kosher salt
1 1/2 pounds kohlrabi, peeled, cut into 1/2" cubes
1 pound russet potatoes, peeled, cut into 1" cubes
1 Granny Smith apple (1/2 pound), peeled, cored, cut into 1" cubes
2 tablespoons (1/4 stick) unsalted butter
Freshly ground black pepper
Fresh chervil sprigs
Special equipment: A potato ricer or food mill
Directions
Add celery root to a large pot of boiling salted water. Reduce heat to medium-low and simmer until tender, 11-13 minutes. Using a slotted spoon, transfer celery root to a large bowl. Return water to a boil; repeat with kohlrabi, then potatoes, cooking each separately until tender, 14-16 minutes for kohlrabi and 10-12 minutes for potatoes; add to bowl with celery root.
Meanwhile, bring apple and 2 tablespoons water to a boil in a small saucepan. Cover and cook over medium-high heat, stirring occasionally, until apple falls apart, 6-8 minutes, adding water by tablespoonfuls if dry.
Working in batches, pass celery root, kohlrabi, potatoes, and apple through a potato ricer into a large bowl. Pass mixture through ricer again if a smoother texture is desired. Stir in butter. Season with salt and pepper. DO AHEAD: Can be made 1 day ahead. Transfer to a microwave-safe bowl, cover, and chill. Rewarm in microwave in 30-second intervals until heated through. Transfer to a large serving bowl. Garnish with chervil sprigs.
