1 package frozen puff pastry (2 pieces)
1/2-3/4 cup goat cheese, at room temperature
1 large or 2 medium zucchini, ends trimmed, sliced lengthwise
1 red pepper, core and seeds removed, cut in half
2 portobello mushroom caps
1 red or sweet onion, peeled, cut into large rings
Kosher salt and fresh black pepper
A few Tbsp of all-purpose flour
1/4 cup grated Parmigiano-Reggiano or Gruyere cheese
1 egg beaten with 1 tsp water
Set the puff pastry on the counter to defrost while you prepare the filling for the tart. Bring the goat cheese out of the refrigerator. Preheat the grill to high. Preheat the oven to 450°F. Line a rimmed baking sheet with parchment paper and set aside.
Place the vegetables on a platter, brush on both sides with olive oil, and season with salt and pepper. When the grill is heated, cook the vegetables for 2-3 minutes per side, until they are cooked through. Set aside to cool slightly.
Sprinkle a counter top with flour, and roll out one sheet of puff pastry. Place it on the prepared baking sheet.
Spread the goat cheese over the pastry, leaving a half-inch margin all the way around. Slice the red pepper and mushrooms, and arrange all of the vegetables in layers on top of the goat cheese. Sprinkle the parmesan cheese on top. Paint the edge of the pastry with the beaten egg.
Roll out the second sheet of pastry, and fit it over the top of the vegetables. Make sure to press down all around the edge to form a tight seal. If needed, use a sharp knife to trim the edge to make it even. Press down on the edge with the tines of a fork, all the way around the pastry, to complete the seal.
Brush (with a pastry brush) the egg wash over the top of the pastry. Wait for one minute, then apply a second coat. With a sharp knife, make three diagonal slits in the top of the pastry; be sure to cut all the way through the pastry.
Place in the oven for 20 minutes or until the pastry is browned and puffed. Remove from the oven, and let cool for 10-15 minutes before serving.