Take your puff pastry sheet from the freezer and let stand on your counter until softened (30 to 45 minutes). Unfold. Sprinkle flour lightly on the surface so you can roll out your pastry to one-third larger than the original. You're just trying to make it a few inches wider than taller.
Flip, sprinkle with flour and roll again. Spray the outside inch of the pastry with the butter spray and fold one-half inch of the pastry in toward the middle all the way around. You are making a border with the pastry. The butter will help it stick.
Fold the pastry in half so you can transfer it to your cookie sheet. Spray the cookie sheet (use one with no sides if possible) with the butter spray first and then transfer the pastry and unfold it so it covers the sheet.Bake at 400 degrees for 15 minutes until lightly golden brown. Remove.
The puff pastry will have puffed up. Just poke it with a fork to let the air out and settle it down with your fork so it flattens out. Let cool. This can sit on the counter for a few hours until getting ready to serve.
When you want to bake the tart, sprinkle the mozzarella cheese on the pastry. (I talk above out a medley of cheese because now some companies make a mozzarella medley that works great. Last time I made this, the medley I used was mozzarella, asiago and roasted garlic. It was perfect. You can find our own combination or just use plain mozzarella.)
Place the tomato slices on the pastry in rows, top to bottom. Large tomato slices usually take 4 slices per row to cover the pastry. I alternate yellow and red tomatoes if I have them. You can assort any way you want or even use varying sizes.
When the tart is covered in tomatoes, sprinkle on the parmesan and the dried herbs and bake at 400 degrees for 15 to 20 minutes more. You're roasting the tomatoes now. Watch at the end so your tart does not get too brown.
Take from oven and let it cool a bit. Gently slide the tart off the cookie sheet onto a cutting board.