Cook onion in hot oil in a large skillet over low heat, stirring often, 30 to 35 minutes or until onion is caramel colored. Stir in salt, and set mixture aside.
Unfold pastry sheet; fit into a 9-inch square tart pan.
Bake at 400° for 15 to 20 minutes or until browned. Remove from oven. Press pastry with the back of a spoon to flatten. Top with caramelized onion; sprinkle with Parmesan cheese. Bake 5 more minutes. Garnish, if desired.