The delicious combination of cheese with balsamic is a rich treat, perfect for sharing.
Melt butter in large skillet. Add onions and salt.
Cook over medium heat, stirring occasionally until onions are amber coloured and caramelized, approximately 20 - 25 minutes.
Add vinegar and thyme. Cook 1 - 2 minutes more and set aside.
Unroll crust onto a baking sheet lined with parchment paper. In the centre of crust place 2/3 of cheese spreading to within 1 1/2 " of edge.
Top with onion mixture.
Fold edges of crust over so that a small circle of onion is still visible in centre of tart.
Beat the egg with water. Brush crust with egg wash until lightly coated. Sprinkle with remaining cheese.
Bake at 425F for 10 minutes. Reduce heat to 350F and bake for additional 15 - 20 minutes or until crust is golden brown.
Let sit about 10 minutes before serving.