Preheat the oven to 375°F (190°C). Roll out the puff pastry sheet onto a baking tray lined with parchment paper.
In a large skillet, heat olive oil over medium heat. Add onion and garlic and cook until softened, about 3 minutes.
Add eggplant, zucchini, red and yellow bell peppers to the skillet. Season with salt, pepper, and dried thyme. Cook until vegetables are tender, about 10-12 minutes.
Spread the cooked vegetables evenly over the puff pastry, leaving a 1-inch border. Sprinkle grated Parmesan cheese on top.
Fold the edges of the pastry over the filling to create a border. Bake in the preheated oven for 25-30 minutes or until the pastry is golden brown.
Remove from oven and let cool slightly before slicing and serving warm.
Notes
This tart can be served as a main dish with a fresh green salad or as an elegant appetizer. Leftovers taste great cold or reheated.