This is best eaten on the day it’s made, but it can keep at room temperature happily for several hours. If you made it a day or two ahead, wrap it in plastic wrap and keep it in the fridge. Bring to room temperature or gently reheat in a 325°F oven for about 4 minutes.
Note: I love the kick of Dijon as a base for the tarts, but if you think it might be too spicy for some in your family, skip it, or use 1 tablespoon and coat a few of the rectangles and leave the others bare under the zucchini.
You could also sprinkle grated Parmesan, Gruyère, or crumbled goat or blue cheese on top for the last couple of minutes of baking. Use about ¾ cup of cheese for the 6 rectangles. And, you can use this as a blueprint recipe for about five million different tarts. It’s that simple, and it’s that flexible. Thaw puff pastry and cut the sheets into thirds, smear them with something delicious, like pesto or mayonnaise or a flavored aioli, layer on some quick-cooking, thinly sliced vegetables, and sprinkle or crumble something else on top, such as chopped nuts or pumpkin seeds.
To create a more defined, puffed-up border on each tart, use a sharp knife to score all around the rectangle about 1/3 inch from the border.
Cut down no more than halfway through the thickness of the uncooked pastry. This will allow the edges to really rise, while the ingredient-covered middle section will remain flatter, though still flaky.
1. Preheat the oven to 400°F. Remove the puff pastry from the freezer and thaw according to the package directions.
2. Unfold both of the puff pastry squares onto a clean surface and cut them into thirds (it will probably be very easy to cut along the folds). You will have 6 pieces. Place them onto 1 or 2 rimmed baking sheets—whatever will fit the pieces without them touching. Use a fork to prick holes all over the pastry, leaving a 1/4-inch border unpricked on all sides (see the Cooking Tip below).
3. You can continue with Step 4 to make the zucchini topping, or see below for some other topping suggestions.
4. Heat the olive oil in a large skillet over medium-high heat. Add the zucchini, season with salt and pepper, and sauté until tender, about 5 minutes. Remove the skillet from the heat and stir in the oregano.
5. Spread each piece of pastry with 1 teaspoon of the Dijon mustard, if using, leaving the borders plain (again about 1/4 inch on all sides). Spread the sautéed zucchini evenly over the mustard, still maintaining the border.
6. Bake until the border of the puff pastry is nicely browned and puffed, 20 to 25 minutes. Transfer the baking sheets to wire racks and let the pastries cool for a couple of minutes, before cutting each into pieces.