Preheat oven to 350°F (175°C). Line a standard 12-cup muffin tin with paper liners. Set aside.

Prepare the crust: In a small bowl, combine graham cracker crumbs and 1/4 cup sugar. Pour in melted butter and mix until well combined. Press about 1.5 tablespoons of the mixture firmly into the bottom of each muffin liner.

Bake the crusts for 5 minutes. Remove from oven and while they cool slightly, prepare the topping and filling.

Prepare the topping: In a small saucepan, combine rhubarb, strawberries, brown sugar, lemon juice, and cornstarch. Cook over medium heat, stirring frequently, until the mixture thickens slightly and the rhubarb softens (about 5-7 minutes). Remove from heat and let cool completely.

Prepare the filling: In a large bowl, beat the softened cream cheese and 3/4 cup sugar with an electric mixer until smooth and creamy. Beat in the vanilla and sour cream. Mix in the eggs one at a time, mixing only until just combined after each addition. Do not overmix.

Fill the liners: Divide the cheesecake batter evenly over the partially baked crusts, filling each liner about 3/4 full.

Bake for 18-20 minutes, or until the edges are set but the centers still have a slight jiggle. Turn off the oven heat, crack the oven door open slightly, and allow the cheesecakes to cool in the oven for 30 minutes to prevent cracking.

Remove from oven, cool to room temperature, then refrigerate for at least 4 hours, or preferably overnight, until firm. Top each mini cheesecake with a spoonful of the cooled strawberry rhubarb topping just before serving.