Place the oven rack in the middle of the oven. Preheat the oven to 200°C/390°F. If rolling out the pastry from a block, roll out until 3 to 5mm thick.
Line a large baking tray or two with baking paper. A half sheet (40cm x 28cm or 16"x11") is the perfect size for a roll of pastry. If using a quarter sheet, the pastry will need to be divided in half for each baking tray.
Crack open an egg, add in a tablespoon of water, and beat until the egg is well mixed. Set aside.
1 egg, 1 tablespoon water
Cut the pastry to fit your baking tray(s). To make the pastry border, slice off strips of pastry about an inch or 2.5cm wide. Set aside.
350 grams puff pastry
Place the large piece of pastry onto the baking tray. Dock the pastry by poking it every inch to 3cm with a fork. Brush the pastry generously with the beaten egg.
Place the border strips on the edges of the docked pastry. Gently press down. Brush the tops of the border with the beaten egg.
Bake the pastry for 15 minutes, rotate the baking sheet if needed half way through baking.
While the pastry is baking, grate the cheese(s) and thinly slice the red onion.
Slice the tomatoes about 3-5mm thick. Place the sliced tomatoes on a piece of paper towel, and then press an additional paper towel on top of the tomatoes to remove some of the moisture from the tomatoes.
3-5 tomatoes
When the pastry has baked for 15 minutes, remove it from the oven. If the middle has puffed up, simply press it down using a fork or spoon.
Sprinkle the cheese over the center of the pastry, then lay the tomatoes on top of the cheese, and then a few slices of red onion. Top with salt and pepper.
Bake the tart for a further 12 to 15 minutes. Rotate the pan if necessary. Remove from the oven. Serve the tart warm or at room temperature. Serve with a green salad to make lunch or dinner.
All temperatures stated are for conventional ovens. Decrease the temperature by 20°C/25°F or gas mark 1 for "fan" or air-fryers.
Notes
The width of my pastry roll was the perfect size for the short length of my pan, but the roll was way too long. So I cut the pastry long enough to fit on my sheet. Then I used the leftover pastry and cut 1-inch strips. The photo beneath step 6 shows that my strips for the long sides were too short, so I just butted two strips against each other and trimmed them for the long edges.
Allow the puff pastry to completely defrost before attempting to roll it open. If the pastry is frozen at all then it will crack when unrolling it.
Prebaking is essential to prevent a soggy bottom.
This tart is best served warm or at room temperature.