For the Heirloom Tomato Tart:

1. Preheat the oven to 400°F. Place the puff pastry on a clean, floured countertop or cutting board, then roll out slightly (to about 11×14”). Using a knife or pizza cutter, lightly score a rectangular border of about 1” in from the edge, being cautious to not cut all the way through. Transfer puff pastry onto a parchment paper lined baking sheet.
2. Whisk egg with a small splash of water and brush the egg wash around the border. Sprinkle the border with everything but the bagel seasoning. Transfer to the oven and bake for 15-18 minutes or until golden brown around the edges and slightly puffed up. Set aside to cool. 
3. While the puff pastry is baking, slice the tomatoes into ¼” slices and place on a paper towel lined baking sheet. Season with salt and let sit for 15 minutes to help draw out moisture.
4. Meanwhile, add boursin cheese to a bowl with the milk and lemon zest. Mix until combined, adding more milk as needed to reach the ideal consistency (you want it the consistency of ricotta).
5. Once the puff pastry has cooled, spread the boursin evenly inside the border of the puff pastry.
6. Pat the tomatoes dry, then arrange on top of the boursin (tomato slices can overlap)
7. Drizzle the tomatoes with olive oil and season with flaky sea salt. Slice into squares and serve immediately.