Thaw the puff pastry according to the package instructions. Preheat your oven to 400F and line a rimmed baking sheet with parchment paper.
In a large bowl, combine the diced rhubarb, sliced strawberries, 1 teaspoon of lemon juice, and 3 tablespoons of cane sugar. Mix well, gently folding the ingredients together so as not to bruise the fruit. Taste a piece of strawberry, and adjust the sweetness with more sugar or lemon juice as needed.
Add 2 tablespoons of cornstarch to the fruit mixture and fold until it’s completely dissolved and the filling appears slightly glossy.
Generously dust a clean work surface with flour. Unfold the puff pastry onto the floured surface and lightly roll it out with a rolling pin, ensuring it’s not too thick.
Transfer the rolled puff pastry to the lined baking sheet. Place the prepared fruit filling in the center of the pastry, using a spatula to spread it into a rough circle.
Fold the edges of the puff pastry over the fruit, pinching the folds together to create a rustic edge.
If you desire a golden and crispy crust, brush the edges with maple syrup and sprinkle with turbinado sugar.
Bake the galette in the preheated oven for 25-30 minutes, or until the edges of the puff pastry rise and turn golden brown.
Once baked, remove the galette from the oven and let it sit for about 10 minutes.
Transfer the galette to a cutting board and slice it into 6-8 pieces. Serve warm, either plain or topped with dairy-free vanilla ice cream or whipped cream for an indulgent treat.