Preheat oven to 375 degrees. Take the thawed pastry out of the fridge so it’s easy to handle when you’re ready to assemble the tart.

Melt the butter in a skillet over medium-low heat, then add the onions. Add in salt and pepper to taste. Cook over medium-low heat until the onions are soft and golden brown (about 20-25 minutes), stirring occasionally.

Place a sheet of parchment paper on a baking tray and unfold the puff pastry onto the parchment paper. Using a fork, pierce the puff pastry in the middle a few times.

Add a layer of the cooked onions followed by a layer of the mixed cheeses.

Slice the tomatoes to medium thickness. Then blot the tomatoes with paper towels to soak up extra moisture. Place the tomatoes on the tart and add salt and pepper to taste. 

Bake the tart for 30-35 minutes. Watch carefully to ensure the crust does not burn. You want the tomatoes to look dark and roasted (some slightly dark areas are ok and delicious) while keeping the tart golden brown.

Take out of the oven and let it sit until completely cool, probably about twenty minutes. Add the basil to the top of the tart while it’s cooling.

Slice and enjoy!