- Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper. Remove the puff pastry from the refrigerator 10 minutes before using to make it more pliable. 
- Slice the tomatoes into 1/4-inch rounds and place them on paper towels. Sprinkle with 1/2 teaspoon of salt and let them sit for 15 minutes to draw out excess moisture. After 15 minutes, pat the tomatoes dry with additional paper towels. 
- Gently roll out the puff pastry on a lightly floured surface to about 10x14 inches. Transfer to your prepared baking sheet. Using a sharp knife, score a 1-inch border around the edge of the pastry (don't cut all the way through). Prick the inner rectangle repeatedly with a fork. 
- Spread the Dijon mustard evenly across the inner rectangle of the pastry. 
- Crumble half the cheese over the mustard layer. Arrange the tomato slices in an overlapping pattern. Sprinkle with minced garlic, thyme leaves, salt, and pepper. 
- Top with the remaining cheese. Brush the border with beaten egg for a golden finish, and drizzle the entire tart with olive oil. 
- Bake for 25-30 minutes, or until the pastry is golden brown and the tomatoes have slightly caramelized. The internal temperature should reach 165°F for optimal texture. 
- Allow the tart to rest for 10 minutes before slicing. If desired, drizzle with balsamic glaze before serving.