- Make the crème fraîche: Mix cream and buttermilk, cover tightly, and leave at room temperature for 24 hours. When ready, it should be thick and creamy. Refrigerate up to 2 weeks.
- Make the filling: Preheat oven to 400 degrees. Toss onion in olive oil, salt, and pepper. Roast for 40 to 50 minutes, until caramelized. Let cool. Rub olive oil over the beet, without peeling, and wrap well in aluminum foil. Roast for 1 hour, until you can pierce it easily with a knife. Submerge in ice water to cool. Skin should peel off easily. Slice into thin discs and set aside. Place figs cut side up on a sheet tray. Roast for 10 to 15 minutes until juices ooze slightly.
- Make the pastry: Roll out dough to the size of a rectangular 12-inch sheet pan. Place on parchment-lined pan. Brush with egg wash, prick all over with a fork, and bake until golden brown. Let cool.
- To assemble: Spread crème fraîche over pastry. Layer beets over the crème fraîche, then the onions, then figs. Sprinkle with sea salt, black pepper, and fresh herbs.