Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper.
Unfold the pastry sheet on your work surface. If the pastry sticks a bit, dust with some flour. Roll the dough out to about a 10-inch square. Cut the square into 4 (5-inch) squares, then cut these in half diagonally to make 8 triangles. Evenly sprinkle all the triangles with the 2 tablespoons sugar. Put the triangles, sugar-side up, on the baking sheet. Pop in the oven and bake until nicely puffed and golden brown, about 25 minutes. Let cool. Slice in half horizontally (a serrated knife is best for this job).
Put a halved triangle on your serving plate. Spread some Strawberry-Rhubarb filling on top, stack another halved triangle and more filling on top of that and repeat to make 4 layers total. Sprinkle with powdered sugar and serve with more sauce on the side. Scoop some Creme Fraiche Ice Cream alongside. Serve the napoleons immediately.
Cook's Note: Both the pastries and filling can be prepared 1 day ahead and stored separately. Put the pastries in an airtight container at room temperature, and cover and refrigerate the filling. Heat up the filling before using.
Whisk the sugar and cornstarch in a small bowl, making sure there are no lumps. Add the rhubarb, sugar-cornstarch mixture, orange zest and juice and half the strawberries to a saucepan. Give it a good stir. Let this mixture sit about 15 minutes off the heat so it macerates (gets nice and juicy). Then put the pan over medium-high heat and let cook until the rhubarb is tender but still holds its shape, giving it a stir now and then, about 7 minutes. Let it cool for about 15 minutes, and then stir in the rest of the strawberries.
Creme Fraiche Ice Cream:
Pour the milk into a large bowl, add the 2 tablespoons lemon juice and let it sit for a couple of minutes.
Add the creme fraiche, the remaining 1/3 cup lemon juice and the sugar. Give it a whisk until thoroughly combined. Pour into an ice cream maker and churn as per the manufacturer's instructions, then pop in the freezer until it's firm. Yield: about 1 quart