Thaw the puff pastry sheet for about 30 minutes.
Preheat the oven to 400℉.
Flour the counter or wood cutting board and a rolling pin so that the pastry does not stick.
Roll out the pastry to a 16-by-10 inch rectangle.
Place the pastry onto a parchment lined baking sheet. (I love the precut parchment paper!)
With a sharp knife, lightly score the pastry dough 1 inch in from the edges creating a border. Do not cut all the way through the dough.
With a fork, pierce the dough at about ½ inch intervals on the inside of the scored line.
Bake until light golden brown, about 10 minutes.
Remove the pastry shell. If it is puffed up in the middle, gently push it down.
Turn oven temperature down to 375℉.
While the puff pastry is baking, sauté the onions in the olive oil over medium heat for about 5 minutes.
Add the thyme, parsley and garlic to the onions and sauté for another 5 minutes. Add salt and pepper to taste.
Sprinkle 1 cup of the mozzarella cheese on the warm puff pastry within the border.
On top of the mozzarella cheese sprinkle ¼ cup of the parmesan cheese.
Sprinkle the onion mixture on top of the cheeses. Use a spatula to get all the spice and olive oil from the pan onto the tart.
Top with the sliced tomatoes, overlapping so they cover the top of the tart.
Sprinkle lightly with salt and pepper.
Sprinkle the top of the tomatoes with the remaining mozzarella and parmesan cheeses.
Bake the tart for 25-30 minutes. Sprinkle with basil and serve warm or at room temperature and ENJOY!