Serve this peppery replacement for mashed potatoes alongside seared steak.
In a medium saucepan, bring turnips, potatoes, and bay leaf to a boil in salted water over high; cook until vegetables are tender when pierced with a knife, 20 minutes. Drain, discard bay leaf, and return vegetables to pan. Add sour cream and butter, and mash until mostly smooth. Season to taste with salt and pepper.