Step 1: Prepare the Crust

1. Preheat Oven: Preheat your oven to 325°F (160°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
2. Mix the Crust Ingredients: In a medium bowl, combine graham cracker crumbs, melted butter, and sugar. Stir until the mixture resembles wet sand.
3. Press into Pan: Press the mixture firmly into the bottom of the prepared pan, creating an even layer.
4. Bake: Bake for 10 minutes, then set aside to cool while preparing the cheesecake layer.

Step 2: Make the Cheesecake Layer

1. Beat the Cream Cheese: In a large mixing bowl, beat the softened cream cheese with a hand mixer or stand mixer until smooth and creamy.
2. Add Sugar and Flour: Gradually add sugar and flour, beating until fully combined.
3. Incorporate Eggs: Add eggs one at a time, mixing well after each addition.
4. Mix in Sour Cream and Vanilla: Stir in the sour cream and vanilla extract until smooth.
5. Pour into Crust: Pour the cheesecake batter over the cooled crust, spreading it evenly.

Step 3: Bake the Cheesecake

1. Prepare a Water Bath: Wrap the bottom of the springform pan in aluminum foil to prevent leaks. Place the pan in a larger baking dish and fill the dish with hot water until it reaches halfway up the sides of the springform pan.
2. Bake: Bake the cheesecake for 45-50 minutes, or until the edges are set and the center jiggles slightly.
3. Cool: Turn off the oven and leave the cheesecake inside with the door slightly ajar for 1 hour. Remove and cool to room temperature, then refrigerate for at least 4 hours or overnight.

Step 4: Prepare the Blueberry Mousse

1. Cook the Blueberries: In a small saucepan, combine blueberries, sugar, and lemon juice. Cook over medium heat until the blueberries burst and the mixture thickens slightly, about 8-10 minutes.
2. Blend and Strain: Blend the mixture using an immersion blender or food processor until smooth. Strain through a fine mesh sieve to remove seeds, if desired.
3. Bloom the Gelatin: In a small bowl, sprinkle gelatin over cold water and let it sit for 5 minutes. Microwave for 10 seconds to dissolve, then stir into the warm blueberry puree. Let cool to room temperature.
4. Whip the Cream: In a chilled bowl, beat heavy whipping cream and powdered sugar until stiff peaks form.
5. Fold Together: Gently fold the whipped cream into the cooled blueberry mixture until no streaks remain.

Step 5: Assemble the Cheesecake

1. Spread the Mousse: Once the cheesecake has fully cooled, spread the blueberry mousse evenly over the top. Smooth with a spatula for an even layer.
2. Chill: Refrigerate for at least 2 hours, or until the mousse is set.

Step 6: Garnish and Serve

1. Garnish: Decorate with fresh blueberries, dollops of whipped cream, or mint leaves for a beautiful presentation.
2. Slice and Enjoy: Carefully release the springform pan, slice, and serve chilled.