How to Make It
Preheat oven to 375°F. Toss beet with oil, a pinch of salt, and several grinds black pepper in a microwave-safe dish. Add rosemary and ¼ cup water. Cover tightly with plastic wrap and microwave until a paring knife slips easily through the flesh, 8-10 minutes. Remove from microwave and let cool in covered dish. Rub off the skins with a paper towel. Alternatively, steam beets in a steamer basket over a pot of simmering water.
Transfer the beet to a food processor and add tahini. Process until smooth. Season with salt and pepper to taste.
Unroll crescent roll dough on a lightly-floured piece of parchment paper. Pinch together any seams so that you have a smooth rectangle with no holes. Flip the dough over and repeat. Use a 3” cutter to cut 7 rounds from the dough. Transfer parchment to a baking sheet.
Brush rounds with beaten egg. Divide poppy and sesame seeds and a small pinch of flaky salt among rounds, being sure to sprinkle seeds to the edges of the rounds. Freeze for 5 minutes.
Flip the rounds over so they are seed-side down. Brush edges with beaten egg. Place 1 to 1½ teaspoons of beet puree in the center of each round (you will have leftover puree). Fold sides up to make a triangle, pinching points to seal and leaving some filling exposed. Return to freezer for 10 minutes (this will help hamantaschen hold their shape in the oven).
Bake until golden brown, 8 to 10 minutes, pressing edges down halfway through baking if they begin to unfold. Serve warm.
Extra beet puree: Serve with cream cheese on a baguette, or stir into hummus or labne for a gorgeous pink dip.