Yield: 8 side or 24 appetizer sided pieces
Frozen puff pastry makes this dish come together quickly - perfect as a Thanksgiving side or a vegetarian entrée. Cutting the squash into thin ribbons allows them to roast while the tart bakes, cutting out an extra step. Serve this in small squares for an elegant appetizer or with a simple green salad for an easy fall lunch.
Remove puff pastry sheet from package and set out to thaw.
In a medium skillet over medium-low heat, melt butter. Add onion slices and ½ teaspoon salt. Cook, stirring occasionally, until well caramelized, about 30-40 minutes. You may need to turn down the temperature to low to prevent onions from browning. When onions are finished, set aside.
While onions cook, heat oven to 400F.
If puff pastry is thawed enough to work with, unfold and place on counter. Using a rolling pan, gently roll pastry until it is about 9x12 inches. Transfer to a baking sheet.
Use a fork to prick pastry sheet all over. Be sure to go all the way to the edges.
Brush the pricked pastry with 2 Tablespoons olive oil.
Sprinkle oiled pastry with shredded Gruyère. Set aside.
Using a vegetable peeler, peel the skin off the squash and discard. Then, peel ribbons of squash and transfer to a bowl. You will need 6 cups of squash ribbons (about half 1 medium squash).
Toss squash ribbons with remaining 1 Tablespoon olive oil, thyme and ½ teaspoon salt until ribbons are coated. Set aside.
Return to the pastry with oil and cheese. Top cheese layer with caramelized onions, doing your best to spread them evenly over the cheese.
Top onion layer with butternut squash ribbons, making sure to cover the pastry from edge to edge.
Bake for 22 minutes, until pastry is golden, cheese is bubbly and squash is started to brown on some edges. Let cool for 5 minutes. Transfer to a cutting board and cut into approximately 2-inch squares. Serve warm or at room temperature.