Alsatian Onion Tart (Zwiebelkuchen)

Preheat oven to 350° F. Place a sheet pan on the lowest rack of the oven. Line a 9” tart pan with crust and dock. Freeze for 30 minutes.Sauté onions in a large skillet over medium-high heat with butter and salt, scraping up the browned bits occasionally, until onions are golden and tender, 20-30 minutes Set aside to cool.Meanwhile, line tart shell with parchment paper and fill with rice, beans, or pie weights. Place tart pan on top of sheet pan and blind bake for 12 minutes. Remove parchment and rice, beans, or weights and bake another 10 minutes until sides of tart are beginning to color and bottom looks cooked. Remove from oven.In a bowl, whisk together egg, heavy cream, Muenster cheese, thyme, nutmeg, and pepper. Add egg mixture to cooled onion mixture, scraping up any remaining brown bits from bottom of pan. Pour onion mixture into tart shell and spread evenly.Bake 30–35 minutes on the sheet pan until filling is golden brown and set. If the edges of the crust brown too quickly, cover edges with foil. Cool 10 minutes and serve hot.