In a mixing bowl, combine all-purpose flour, old-fashioned oats, brown sugar, salt, and optional ground cinnamon. Gradually pour in melted unsalted butter, mixing until the mixture resembles coarse sand.

Preheat your oven to 350°F (175°C). Line an 8x8 or 9x9-inch pan with parchment paper. Press two-thirds of the crumble mixture into the bottom of the prepared pan, then bake for 10 minutes until lightly golden.

In a large mixing bowl, beat the softened cream cheese until smooth. Gradually add granulated sugar, then add the egg and vanilla extract, mixing gently until incorporated.

In a separate bowl, toss chopped rhubarb stalks with granulated sugar and cornstarch until evenly coated.

Pour the cream cheese filling over the pre-baked crust, then spoon the rhubarb mixture on top, before sprinkling the remaining crumble mixture over.

Return the assembled bars to the oven, baking at 350°F (175°C) for 35-45 minutes. The center should be set and the top lightly golden.

Allow them to cool in the pan for about 30 minutes, then transfer to the refrigerator to chill for at least 2 hours.