Cut cherry tomatoes in half, sprinkle salt over the tomatoes and let it rest for 30-35 minutes to let the water drain. Blot out excess water from the tomatoes with paper towels.
Heat skillet over medium heat and add butter. Once butter is melted, add onions and olive oil. Sauté onions for 25-30 minutes or until onions are browned. Stir in 2 teaspoons of fresh thyme leaves and set aside to cool.
Preheat oven to 400°and line baking sheet with parchment paper. Place puff pastry sheet on a floured surface and roll out the dough to approximately 7½ x 12" rectangle.
Transfer the rolled out dough on the baking sheet and make a 3/4 inch border with a knife, careful not to cut through the dough.
Sprinkle grated parmesan cheese evenly over the pastry dough, then spread caramelized onions on top. Don't spill the ingredients onto the pastry border.
Next, arrange tomatoes on top of onions and sprinkle grated comté or gruyere over the tomatoes and finish off with sprigs of thyme, salt and pepper.
Brush the border with egg wash and bake the tart for 18-20 minutes or until the puff pastry edges are golden brown.
