In a heavy non-reactive saucepan, combine mulberries, sugar, and lemon juice and stir well. Put the pan over medium heat and cook, stirring frequently until the mulberries are soft and the liquid they release has reduced somewhat, about 10 minutes.
Let it cool slightly then transfer the mulberries and their juice to a blender or food processor and puree until smooth. Strain the mixture through a fine-mesh strainer into a bowl, pressing on the solids to extract as much puree as possible. Discard the solids and refrigerate the mulberry.
In a medium heatproof bowl, whisk the yolks just to break them up and whisk in ¼ cup sugar. Set aside.
In a heavy non-reactive saucepan, combine cream, milk, salt and ½ cup of sugar.
Put the pan over medium heat and let the mixture boil gently to bubbling just around the edges (gentle simmer). Reduce the heat to low.
Carefully measure out ½ cup of hot cream mixture. Whisk in the hot cream mixture while whisking the eggs constantly. Continue tempering the eggs by adding another ½ cup of hot cream to the bowl with the yolks.
Pour the cream-egg mixture back to the saucepan and cook over medium heat, stirring constantly until it is thickened and coats the back of a spatula, about 5 minutes.
Strain the base through a fine-mesh strainer into a clean container.
Pour the mixture into a 1-gallon Ziplock freezer bag and submerge the sealed bag in an ice bath until cold, about 30 minutes. Refrigerate the ice cream base for at least 2 hours or overnight. I like to refrigerate the base overnight for the most flavor.
When ready to churn the ice cream, whisk the creme fraiche, then the mulberry puree into the ice cream base.
Pour the ice cream base into the frozen canister of your ice cream machine and follow the manufacturer’s instructions.
Spin until thick and creamy about 25-30 minutes.
Pack the ice cream into a storage container, press a sheet of parchment directly against the surface and seal with an airtight lid. Freeze in the coldest part of your freezer until firm, at least 4 hours.
To make the crumble, in a medium prep bowl, combine flour, sugar, butter, salt, and cinnamon. Use your fingers and rub the mixture together until it resembles breadcrumbs. Refrigerate the crumble for 15 minutes until chilled.
When ready to bake, preheat the oven to 350 degrees F.
Spread the chilled crumble mixture on a baking sheet lined with parchment paper. Bake for 12-15 minutes or until golden brown.
Transfer the baking sheet to a wire rack and let it cool completely before breaking it into small pieces. Store the crumble in a tight-lidded container until ready to serve.
When ready to serve, scoop the mulberry ice cream into a bowl and top with the crumble.