Place all berries, vanilla, sugar, and lemon juice in a medium pot, and bring to a boil. Lower heat and add the dissolved cornstarch. Simmer 10 minutes. Let cool.

Take vanilla ice cream out of freezer and defrost for a few minutes. Mix in lemon zest. Return to freezer.

Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius).

Place all crumble ingredients into a medium bowl and rub well with your fingers until small, even crumbs are formed.

Place the crumbs on a parchment-lined baking sheet and bake for 10 minutes. Allow to cool.

Prepare individual parfait glasses. Put a few spoonfuls of blueberry compote into each glass. Add a scoop of lemon ice cream and top with crumble.