> 3 tbsp unsalted butter, plus more for brushing
> 2 large sweet onions, thinly sliced
> 2 thyme sprigs, leaves removed and stems discarded
> 1/2 (15 oz) refrigerated pie crust
> 3/4 cup Gruyère cheese, grated
In a 12-inch skillet, melt the butter on medium-high. Add the onions and season with salt. Cook 20–25 min., until caramelized, stirring occasionally. If onions brown too quickly, reduce heat and add a splash of water to the pan. Stir in the thyme leaves and cook 30 sec.–1 min. Remove from heat.
Meanwhile, preheat oven to 400°F. Line a large baking sheet with parchment. Unroll the pie crust and place on baking sheet.
Spread onion mixture over crust, leaving a 1½-inch border. Top with the cheese. Fold the edges over filling and brush the edges with more melted butter. Bake 20–25 min., until crust is golden brown.
If desired, garnish with 2 tbsp chopped chives.