You will need about 4 cups of fruit to generously fill this tart. We used six colors to get the rainbow effect, but you can certainly use one or two or three, depending on what you can find.
Preheat oven to 375 degrees. Lightly dust work surface and puff-pastry sheet with flour; roll out to slightly thinner than 1/4 inch. Sprinkle with 2 tablespoons sugar, pressing and rolling sugar so as much sticks as possible. Trim pastry sheet to a 12-by-8 1/2-inch rectangle. Transfer to a baking sheet. With a paring knife, lightly score a border 1 inch from edges. Freeze 15 minutes. Bake until puffed and golden, 30 to 40 minutes, rotating halfway through. Let cool. Gently press center to flatten.
Meanwhile, place each color of fruit in its own bowl (or do in batches). Divide remaining 3 tablespoons granulated sugar and lemon juice evenly among bowls and add a pinch of salt to each. Toss to combine. Whisk cream with confectioners' sugar until stiff peaks form. Fill pastry with cream and top with fruit. To create rainbow effect, arrange fruit in vertical stripes, starting with red, followed by orange, yellow, green, blue, and purple. Garnish with mint, if desired. Serve immediately, or refrigerate, covered, up to 2 hours.