Preheat oven to 425°F. Line two baking sheets with parchment paper, set aside.
In a large bowl, toss together the sliced strawberries, rhubarb, brown sugar, flour and cinnamon with a pinch of salt. Set aside.
In a large bowl, beat together the cream cheese, sugar, flour, 1 egg, vanilla, lemon juice and cinnamon until smooth. Transfer the mixture to a large piping bag, fitted with a large plain circle tip; set aside.
Unwrap both sheets of puff pastry, roll it out slightly and then cut each sheet into six even rectangles so you end up with twelve pieces in total. Place six on each prepared baking sheet. Using a sharp pairing knife lightly score an outline on each piece of pastry along the edge to create a border. Do not cut all the way down through the pastry, just slightly. Then use a fork to poke holes throughout the center of each. Place the sheets in the freezer for about 10 minutes to rechill.
Meanwhile, make the topping. In a small bowl, combine the flour, rolled oats, sugar, cinnamon, and salt. Add the butter and work it in with your fingers until coarse and resembles chunky sand.
Whisk the remaining egg with a splash of water and brush the pastry sheets liberally with the egg wash. Pipe out about 2 tablespoons of filling onto the center of each. Then top with the strawberries and rhubarb. Sprinkle liberally with the crumb topping and bake until puffed up and deep golden brown, about 20 to 25 minutes. Remove from the oven and let cool before serving. Sprinkle with powdered sugar and enjoy!