Preheat oven to 400 degrees F.

Wash and peel the potatoes. Using a mandoline slicer or knife, cut into rounds about 1/8-inch thick. Set slices aside.
Wash and peel the beet. Slice the same way as the potatoes, put into a second bowl. (Keep the potatoes and beets separate to keep the beets’ color from staining the potato slices for a prettier finished product.)

Heat a large, heavy oven-proof skillet on medium high. When skillet is hot, add 2 Tbsp. olive oil and use a pastry brush to coat the sides.

Begin to build the galette in the hot skillet by layering potato slices in overlapping circles until the bottom of the pan is covered. When the first layer is done, sprinkle with a little chopped rosemary, salt and pepper, and 2 Tbsp. grated cheese. Continue with the second layer, this time overlapping alternating slices of potato and beet. Save big, uniform slices for the third and final layer. (Getting the pan hot before putting in the vegetables helps crisp the bottom.) Sprinkle with more rosemary, salt, pepper, and another 2 Tbsp. grated cheese. Continue layering the potatoes and beets until all are used. Add remaining rosemary and grated cheese. Drizzle the top of the galette with remaining 2 Tbsp. olive oil.

Cover pan with aluminum foil, pressing it tightly onto the vegetables, turn the heat to medium, and cook on the stovetop about 10 minutes. You should hear the bottom of the galette sizzling; if not, turn up heat slightly.

Remove foil and place skillet in the hot oven. Bake uncovered about 40-55 minutes, until top is browned, edges are crisp, and vegetables are cooked through (test by inserting the tip of a small knife straight down – you should feel no resistance).

When thoroughly cooked, remove from oven and let sit 5 minutes. Using a heat-proof spatula, loosen it from the pan. Slide the galette onto a cutting board and slice into wedges. Serve immediately.