• Thaw frozen puff pastry until you can unfold it. Keep thawed in refrigerator until just before you need it. Preheat oven to 400 degrees.
• Make an egg wash by beating one egg with one tablespoon water. Set aside.
• Slice tomatoes to ½ inch thickness. For the perfect way to remove the excess water in the tomatoes, place on folded paper towels for 30 minutes. Repeat on the opposite side if needed.
• Slice onion and sauté in a medium skillet with 1 tablespoon olive oil. You are not going for caramelized onions. When the onions are lightly golden brown remove them to a small bowl.
• Grate the cheeses on a box grater.
• Remove the thawed puff pastry from the fridge and unfold it. Crimp the seams together with your fingers. Use a rolling pin to to slightly flatten and seal the seams.
• You can choose to make a slab tart on a baking sheet or a round one in a tart pan.

For a slab tart on a sheet pan:

• For a slab tart: Place dough on parchment paper-lined baking sheet. Use a pizza cutter or sharp knife to remove a half inch strip of puff pastry dough. Place these strips on the edges of your puff pastry to create a well to keep the filling in. (See photo below.) Before laying the strips of dough on the base, moisten the edges by lightly brushing with egg wash. If you are making a slab tart, make your base about 9"x9" square. My tart pan is 9" in diameter.

For a tart in a tart pan:

• If you are making your tart in a pan, cut parchment paper to fit the tart pan. Use the parchment paper as a template to cut the dough in a circle. You want to add a half inch all the way around to make a bit of a lip when you place the dough in the pan.
• Once your puff pastry is seated on parchment paper, whether on a sheet pan or in a tart pan, use a fork to poke holes in the bottom of the pastry dough. This is called docking and it will keep the tart from forming large air pockets. IMPORTANT!
• Lightly brush the dough with Dijon mustard.
• Place the dough in the oven for approximately ten minutes, or until it is lightly golden brown on top. (refer to photo below) Remove from oven and allow to cool. This is good time to check your tomatoes and perhaps flip them over on dry paper towels.
• When the crust is cool add most of the Gruyere and Parmesan cheeses, reserving the remainder.
• Arrange the onions evenly over the cheese. Tear pieces of basil and place on top. Arrange the tomato slices on top in a single layer, not overlapping.
• Evenly sprinkle the tart with the salt, pepper, and dried herbs. Sprinkle the remaining Gruyere and Parmesan cheese on top.
• Bake the tart in a 400 degree oven. Allow tart to bake for 30 minutes. If the edges start to get too brown, cover them with strips of aluminum foil.
• Remove the tart from the oven after 30 minutes. Use a scrunched up paper towel to absorb any standing liquid on the top of the tart. Excess water from the tomatoes will make the otherwise flaky pastry soggy.
• Put the tart back in the oven for an additional 12-15 minutes of bake time. 
• Remove tart from the oven and allow to cool until just warm. This is the best time to eat this tart. You can eat it hot, warm, room temperature or cold as well, but warm is best.
• Use a pizza cutter to cut into desired slabs. I like a big slab but smaller pieces make great appetizers.