1. Adjust oven rack to the middle position. Preheat the oven to 425 degrees Fahrenheit.

Par bake the puff pastry:

1. Line a rimmed baking sheet with parchment paper.
2. Unfold puff pastry in the center of the rimmed baking sheet. If there are two sheets in your package, use two separate baking sheets or place them side by side, leaving a 1 inch space in between the two sheets.
3. Use a sharp paring knife to gently score a ½ inch border around the perimeter of the puff pastry, be sure not to cut completely through the pastry.
4. Use the tines of a fork to pierce holes throughout the portion of the puff pastry inside the border you just made.
5. Bake the pastry for 15 minutes. Remove the pastry from the oven and reduce the heat to 350 degrees.
6. Use the tines of a fork to poke down any puffy areas in the center of the pastry.
7. Return the pastry to the oven and continue to bake until it’s light golden brown, another 13-15 minutes.
8. Remove the pastry from the oven and set aside to cool slightly.
9. Increase the oven temperature back up to 425 degrees.

Prepare the tomatoes:

1. While the puff pastry is baking, prepare the tomatoes.
2. Line a rimmed baking sheet with two layers of paper towels. Lay the tomato slices on top of the paper towels and sprinkle both sides of the tomato slices with 1 teaspoon kosher salt. Let the tomatoes sit for 30 minutes.

Assemble the tart:

1. Using the back of a spoon, spread the whole grain mustard onto the par baked puff pastry.
2. Evenly sprinkle the shredded gruyere over the puff pastry.
3. Use a couple new sheets of paper towels and blot the salted tomato slices.
4. Shingle the tomato slices on top of the cheese, allowing the tomatoes to overlap just slightly.
5. In a small bowl, combine the garlic and olive oil.
6. Spoon some garlic oil over each tomato slice.
7. Top the tomatoes with some fresh cracked black pepper.

Bake the tart:

1. Bake the tart until the cheese has melted and the puff pastry is golden brown, 15-20 minutes. Rotate the pan halfway through baking.

Finish the tart:

1. Remove the tart from the oven, and transfer to a wire rack for 10 minutes.
2. Transfer the pastry to a cutting board, top with basil and flaked salt (if using).
3. Cut the tart into desired sized slices and serve. Can be served warm or at room temperature.