Preheat oven to 350ºF. To toast hazelnuts, place in a single layer in a baking dish and place in oven while its pre-heating. You'll know when they're done because you can smell them and they'll start to turn golden brown. Toasting should only take 5-10 minutes. Once you remove them from the oven, rub as much of the skin off as you can.
Pulse crumble topping ingredients in a food processor until a coarse crumb forms. Set aside. Combine filling ingredients in a medium bowl until well blended. Spread filling in a 9” x 13” baking dish.
Top with crumble. Place on middle oven rack and bake 30 to 40 minutes until cooked through and crumble is browned.
Serve warm with whipped cream, crème fraîche or ice cream.