In a large mixing bowl, combine all-purpose flour, old-fashioned oats, brown sugar, salt, and a sprinkle of cinnamon. Pour in melted unsalted butter and mix until the mixture resembles coarse crumbs. Set aside about 1/3 of the mixture for the topping.

Line an 8x8 or 9x9-inch baking pan with parchment paper and press 2/3 of the crust mixture firmly into the bottom of the pan. Bake in a preheated oven at 350°F (175°C) for 10 minutes.

In a medium bowl, beat softened cream cheese with granulated sugar until fully smooth and creamy. Add in the egg and vanilla extract, mixing gently until just combined.

In another bowl, toss the chopped rhubarb with additional sugar, cornstarch, and fresh lemon juice. Let this sit for a few minutes.

Spread the cream cheese mixture evenly over the pre-baked crust in the pan. Layer the prepared rhubarb mixture on top, distributing it evenly. Sprinkle the reserved crumble mixture over the rhubarb.

Return the assembled bars to the oven and bake at 350°F (175°C) for 35 to 45 minutes until golden brown.

Allow the bars to cool in the pan for 30 minutes, then transfer to the refrigerator and chill for at least 2 hours before serving.