6 tablespoons cold unsalted butter, cut into small pieces
4 pints raspberries
1 lemon, zested and juiced
2 teaspoons cornstarch
3⁄4 cup raw hazelnuts
1 cup heavy cream
1 cup whole milk
6 egg yolks
Make the crumble topping by mixing the flour, flax seeds, brown sugar, 3 tablespoons of the granulated sugar, and salt in a bowl. Using your hands or a fork, work the butter into the flour until the butter is the size of rolled oats. Chill this mixture in the refrigerator.
In another bowl, mix the raspberries with 1⁄4 cup of the sugar, lemon zest, juice, and cornstarch, tossing to evenly coat. Set aside for 10 to 15 minutes. Preheat the oven to 350°F. Place the fruit mixture in a 4-quart baking vessel and cook for 15 minutes.
Remove from the oven and sprinkle the crumble topping over the fruit and return to the oven for 10 to 15 minutes or until the topping has browned. Let sit for 10 minutes before serving.
To make the ice cream, toast the hazelnuts on a baking sheet in a 350°F oven until they start to give off a nutty aroma, being careful not to burn them. Cool the nuts slightly and then crush them using a rolling pin. To make the ice cream base, heat the cream, milk, and remaining 1⁄2 cup of sugar in a heavy-bottomed saucepan to nearly boiling.
Remove from heat. In a bowl, add the yolks. Using a ladle, slowly add some of the hot cream mixture to the yolks while whisking them. Continue to ladle in small amounts of the cream until you’ve added about a cup of the hot mixture and the yolks have tempered.
Pour the yolk mixture back into the pot of cream, this time whisking the cream. Place the combined mixture over low heat and stir continuously with a rubber spatula until it reaches 180°F using an instant-read thermometer or until the mixture thickens enough to coat the back of a spoon.
Pour the ice cream base over the toasted and crushed hazelnuts and let chill over a bowl of ice or in the fridge. Add the mixture to an ice cream machine and process according to the instructions provided by the manufacturer.