Preheat oven to 400°F. Line a baking sheet with parchment paper.
Place the bacon strips (without overlapping) in a cold pan. Cook the bacon over medium heat, low and slow, occasionally turning, about 10 to 15 minutes, until crispy. Transfer to a paper towel-lined plate or cooling rack to drain. Once cooled, crumble the bacon and set aside.
Add the butter to the pan with the bacon grease, and melt over medium heat. Add the onions and season with remaining ¼ teaspoon of salt and sauté for 30 to 35 minutes, stirring occasionally until golden brown and caramelized. Stir in the dates, apple cider vinegar, 2 tablespoons of honey, and fresh thyme. Bring the mixture to a simmer and cook for just about a minute until reduced and sticky.
Unfold a sheet of puff pastry on a lightly floured surface and lightly roll to a smooth 9-inch rectangle. Place the puff pastry sheet on the prepared baking sheet. Using a sharp paring knife, score a ¼-inch-wide border around each pastry. Prick the pastry inside the score lines with the tines of a fork. Place the tart in the freezer for 15 minutes to firm up.
Combine goat cheese, cream cheese, 2 tablespoons of honey, olive oil, ¼ teaspoon salt, and black pepper in a food processor. Process until smooth, scraping down the sides of the bowl as needed.
Remove the tart from the freezer. Spread the whipped goat cheese mixture over the tart, leaving a ½-inch border on all sides, and top with the onion and sticky date mixture. In a small bowl, combine the egg and water. Brush the edges of the tart with the egg wash. Transfer the tart to the oven and bake until the crust is golden about 15 to 20 minutes.
Finish with the crumbled bacon and a sprinkle of flaky salt. Cut into squares. Enjoy!
