1. Coat a 9x9 inch baking pan with cooking spray.
2. On a lightly floured surface, roll out the thawed sheet of puff pastry to 9x9 inches.
3. In a large saucepan over medium heat, combine the strawberries, rhubarb, sugar and salt. Heat, stirring gently now and then, until the fruit is tender and bubbling, about 7 minutes.
4. In a small bowl, combine the lemon juice and cornstarch. Stir into simmering fruit. Simmer, stirring constantly, until thickened, about 1 minute.
5. Preheat oven to 400°F.
6. Spoon the prepared mixture into the prepared pan.
7. Place the pastry over the top, then sprinkle with 1 TBSP sugar. Using a paring knife, cut a couple of vent holes in the pastry.
8. Bake for 25-30 minutes, or until the pastry is golden brown.
9. Spoon into bowls and serve warm or at room temperature.