Preheat the Oven: Preheat your oven to 375°F (190°C). Lightly grease a 12-cup muffin pan with cooking spray or butter.
- Prepare the Pastry: Unroll the puff pastry sheet on a lightly floured surface. Cut the pastry into 12 even squares, large enough to fit into the muffin pan.
- Form the Tarts: Press each square of puff pastry into the bottom and up the sides of the muffin cups. Trim any excess pastry to fit neatly inside.
- Prepare the Filling: In a small bowl, combine the shredded mozzarella cheese, grated Parmesan cheese, chopped basil, salt, and pepper. Stir to combine.
- Assemble the Tarts: Spoon a small amount of the cheese mixture into each pastry-lined muffin cup, then layer 2-3 thin tomato slices on top of the cheese mixture.
- Add Olive Oil and Seasoning: Drizzle each tart with a small amount of olive oil, and if desired, add a splash of balsamic vinegar.
- Brush with Egg Wash: Brush the edges of the puff pastry with the beaten egg for a golden finish.
- Bake the Tarts: Place the muffin pan in the oven and bake for 18-22 minutes, or until the puff pastry is golden and the tomatoes are tender.
- Garnish and Serve: Allow the tarts to cool for a few minutes before removing from the muffin pan. Garnish with fresh thyme or extra basil before serving.