Line a baking sheet with parchment paper. Place the thawed puff pastry sheet on top and lightly smooth it out, being careful not to overwork it.
In a bowl, whisk together the egg and water. Using a brush, brush along a 1-inch perimeter of the puff pastry. If desired, use a fork or knife to lightly score alongside the edge of the egg wash. Use a fork to lightly prick all over the inside rectangle of the tart. Do not prick the edge you brushed with the egg wash!
Spoon the whipped ricotta over the center of the puff pastry. Allow yourself about half an inch of space between the ricotta edge and the egg wash as the ricotta will spread while it cooks.
Arrange the fruit on top of the ricotta. If desired, drizzle the remaining 2 teaspoons of maple syrup all over the fruit. Sprinkle lightly with flaky sea salt.
Transfer to the oven for 30 minutes.