Heat butter over medium-high heat. Add Vidalia onion. Cook 20 minutes, stirring until browned. Add thyme leaves and salt.
Unfold puff pastry sheet onto greased nonstick baking sheet and prick with a fork. Bake at 400 degrees F for 10 minutes.
Spread onions over pastry, then top with Gruyere cheese. Bake at 400 degrees F for an additional 10 minutes. Cut into squares and serve.