In a sauce pan melt butter, add shallots and cook until caramelized, about 8-10 minutes. Cook vegetables in saucepan of boiling salted water until tender, about 20 minutes. Drain. Transfer to heavy-duty mixer fitted with whisk attachment. Beat potatoes and celery root until mashed. Add cheese, shallots and butter; beat until butter melts. Mix in milk. Season with salt and pepper.
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