Preheat the Oven: Preheat your oven to 400°F (200°C) and line two large baking sheets with parchment paper.
Sauté the Aromatics: Heat the extra virgin olive oil in a large skillet over medium heat. Add the minced garlic and chopped red bell pepper, sautéing for 3 minutes until softened.
Add the Vegetables: Stir in the diced zucchini and quartered cherry tomatoes. Cook for an additional 3 minutes.
Wilt the Greens & Season: Add the fresh baby spinach and dried oregano. Season lightly with salt and black pepper. Sauté until the spinach is fully wilted and any liquid in the pan has completely evaporated. Remove from heat and let cool for 5 minutes, then gently fold in the crumbled feta cheese.
Cut the Pastry: Unroll the cold puff pastry sheets onto a lightly floured surface. Cut each sheet into 6 equal squares or circles to yield 12 total pieces. Place the cut pieces onto the prepared baking sheets.
Score and Dock: Using a small paring knife, gently score a light border 1/4-inch from the edge of each pastry piece. Do not cut all the way through the dough. Prick the inside center area a few times with a fork to prevent it from rising.
Assemble the Tartlets: Spoon roughly 1 tablespoon of the cooled vegetable mixture into the center of each pastry square, keeping the filling strictly within the scored border lines.
Bake Until Golden: Brush the outer borders of the pastry with the prepared egg wash. Bake in the preheated oven for 15 to 18 minutes, or until the edges are puffed up high and beautifully golden brown. Remove from the oven and serve warm or at room temperature.