Prepare your baking pan: Preheat oven to 350°F (175°C). Line a 9×9 inch baking pan with parchment paper, leaving overhang on the sides for easy removal later.
Make the graham cracker crust: In a medium bowl, combine graham cracker crumbs, sugar, cinnamon, and melted butter until the mixture resembles wet sand. Press firmly into the bottom of the prepared pan in an even layer. Bake for 8 minutes, then remove and cool while preparing the filling.
Prepare the strawberry-rhubarb filling: In a medium saucepan, combine strawberries, rhubarb, sugar, and lemon juice. Cook over medium heat until the fruit begins to break down and release juices, about 5-7 minutes. Add the cornstarch slurry, stirring constantly until the mixture thickens, about 1-2 minutes more. Remove from heat and let cool slightly.
Make the cheesecake layer: In a large bowl, beat the cream cheese and sugar with an electric mixer until smooth and fluffy, about 2 minutes. Add eggs one at a time, beating well after each addition. Mix in vanilla extract and sour cream until just combined. Do not overmix.
Prepare the streusel topping: In a medium bowl, whisk together flour, brown sugar, oats, and cinnamon. Cut in the cold butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs with pea-sized pieces of butter throughout.
Assemble the bars: Pour the cheesecake batter over the pre-baked crust, spreading evenly. Carefully spoon the strawberry-rhubarb mixture over the cheesecake layer, gently swirling with a knife for a marbled effect. Sprinkle the streusel topping evenly over everything.
Bake the bars: Bake at 350°F for 40-45 minutes until the center is just set and the streusel is golden brown. The center may still have a slight jiggle but will firm up as it cools.
Cool completely: Allow the bars to cool at room temperature for 30 minutes, then refrigerate for at least 3 hours or overnight before cutting. Use the parchment paper overhang to lift the entire dessert out of the pan for easier cutting.