When it comes to tart making, frozen puff pastry is truly a miracle ingredient. Just be sure to thaw the pastry according to package directions. Should you wish to cut a few calories from this cheesy fresh tomato tart, you may use Neufchatel cheese instead of full-fat cream cheese.
Preheat oven to 400 degrees F. Line a large baking sheet with parchment paper. Arrange tomato and onion slices on the prepared baking sheet. Brush with oil; sprinkle with garlic, salt, and pepper. Bake for 20 minutes. Sprinkle with 1/3 cup of the Parmesan cheese. Bake about 3 minutes more or until cheese is melted; cool. In a storage container layer tomato and onion slices between sheets of waxed paper; cover and chill overnight.
Line another large baking sheet with parchment paper; set aside. On a lightly floured surface roll puff pastry into a 13x9-inch rectangle. Use a sharp knife or pastry wheel to cut a 1-inch strip off each side of the pastry. Place pastry rectangle on the prepared baking sheet.
In a small bowl combine egg and the water. Brush edges of pastry rectangle with egg mixture; place pastry strips on edges of rectangle, trimming to fit. Brush pastry all over with egg mixture. Prick bottom of pastry with a fork. Bake for 12 to 15 minutes or until golden brown. If bottom is puffed, gently and carefully press down with the back of a long-handled spatula. Transfer to a wire rack; cool. Transfer pastry to a storage container; cover and chill overnight.
In a medium bowl combine cream cheese, 1/3 cup salad dressing, and the remaining 1/3 cup Parmesan cheese. Beat with an electric mixer until smooth. Stir in 1 tablespoon parsley. Cover and chill overnight.
To serve, spread cream cheese mixture over bottom of pastry (let mixture stand at room temperature for 30 minutes if too firm to spread). Top with tomato and onion slices, overlapping as necessary. Sprinkle with basil and/or additional parsley. Cut tart into 16 pieces. Return tart to storage container; cover tightly.
Tote tart and, if desired, additional salad dressing in an insulated cooler with ice packs. Serve tart with the additional dressing.