Rounds of russet potato, sweet potato, and beet stand on edge in a garlicky béchamel and finish under melted Gruyère for a striking, colourful centerpiece that's lighter than a classic cream gratin but every bit as rich-tasting.

A mandoline is essentially non-negotiable here — you'll want very thin, uniform rounds (about ⅛ inch) so they stand up evenly and cook at the same rate, and slicing the beets just a touch thinner than the potatoes (they're denser) helps everything finish cooking together.