1. Take the puff pastry out of the freezer and place it in the refrigerator.
2. Heat oven to 400 degrees Fahrenheit. Line a baking sheet with parchment paper.
3. In a large skillet, heat olive oil. Add onions, salt, and pepper. Cook on medium-high heat for seven minutes until the onions start to soften. Stir frequently.
4. Add sugar, two teaspoons of the white grape reduction, vinegar, and half a cup of water. Continue cooking for an additional 10 minutes, stirring frequently. The onions should be simmering.
5. Reduce heat to medium and keep cooking the onions for 10 more minutes, or until they turn a golden color. You are looking for a medium-light golden color, not a dark brown. Time will vary depending on your stove and type of pan used. Continue stirring.
6. Add thyme and mix well.
7. Remove puff pastry from the refrigerator and placed on the prepared baking sheet.
8. Leaving a half-inch border around the puff pastry, spread the mustard evenly on the puff pastry.
9. Place the cooked onions on top of the mustard.
10. Brush the plant-based milk on the exposed puff pastry border. This step is optional but recommended since it gives the puff pastry a shiny golden color.
11. Place the baking sheet in the freezer for 10 minutes.
12. After 10 minutes, move the baking sheet directly from the freezer into the oven. Bake for 25 to 30 minutes, or until the tart is golden brown.
13. Serve warm and drizzle the remaining two teaspoons of wine grape reduction on top of the onions right before serving.
14. Cut into nine squares, about three inches x three inches.
Make ahead instructions: The onions can be cooked two to three days ahead of time. When ready to serve, place onions on the puff pastry sheet and bake.
Freezing instructions: Freeze the tart whole before baking and bake when ready to serve. Wrap tart with plastic wrap and aluminum foil and place in a sealed freezer bag for up to one month.
Cooked onions can also be frozen for up to a month, just defrost before making the tart.
Reheating: Place already cooked tart in a 200-degree-Fahrenheit oven until warm, about 10 minutes.
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