In a large saucepan, combine raspberries, cherries, granulated sugar, lemon zest, lemon juice and water over medium-high heat. Bring mixture to a boil, then reduce heat and simmer for 5-7 minutes. Remove mixture from heat and let stand 5 minutes. Pour mixture into a blender and process until smooth. Strain mixture into a large bowl using a fine-mesh sieve. Wipe saucepan to remove any residue (like raspberry seeds) and pour strained mixture back into the saucepan.
In a small bowl, whisk egg yolks and cornstarch together until smooth.
Gradually stir egg mixture into fruit mixture. Bring to a boil over low-medium heat, then cook for an additional minute while stirring constantly. Once mixture coats the back of a spoon, remove from heat. Gradually stir in butter until smooth.
Pour filling into a medium bowl and cover with plastic wrap (press the wrap directly onto the filling to avoid a film forming). Chill for 2 hours.
For the Crust
Preheat oven to 350°F.
In a large bowl, add graham cracker crumbs with melted butter and stir until evenly combined. Press a thin layer of crumb mixture into the bottoms of 6 mini tart cases, making sure to go up sides as well.
Bake for 10 minutes. Remove from oven and cool completely.
For the Assembly
Spoon chilled raspberry cherry filling onto cooled mini tart bases. Spread filling evenly. Garnish with fresh cherries and raspberries. Chill in refrigerator for 2 hours prior to serving.