In a mixing bowl, combine the flour, quick oats, brown sugar, salt, and baking soda. Stir together. Melt the butter and add to the dry ingredients. Mix well until the butter is fully absorbed and the mixture is crumbly. Set aside.
Raspberry Apple Filling
Peel, core, and thinly slice the apples. Combine the apples and raspberries in a mixing bowl and drizzle with the lemon juice. Add the sugar, flour, cinnamon, and nutmeg and toss together until the dry ingredients fully coat the fruit and there is no more loose powder.
Assemble and Bake
Preheat the oven to 350°F. Spray a 9-inch tart pan with a removable bottom with non-stick spray. Add about two thirds of the crumble mixture to the tart pan. Press the crumble up the sides and over the bottom of the tart pan.
Spoon the filling into the crust, pressing down gently to pack it lightly into the crust. The filing will be higher the edges of the tart, just smooth it into a rough dome shape. Crumble the remaining crumb mixture over the filling, pressing it gently and evenly over the fruit.
Place the tart pan on a baking sheet and transfer to the oven. Bake for 45 to 55 minutes until the crust is browned and the apples are soft. Use a paring knife to check that the fruit is fully cooked inside. If the crumble begins to darken too much while the fruit is still hard, cover loosely with foil and continue baking until the apples are tender.
Allow the tart to cool for at least 30 minutes in the tart pan. If you try to remove it while still hot, crumble crust might break apart. Once the tart has cooled and tart crust is firmer, gently remove the sides of the tart pan. Use a thin spatula or flipper to separate the bottom of the tart from the pan and slide onto a serving platter. Let cool.
To serve, whip the cream. Combine the whipping cream and sugar in a mixer and whip until it forms soft peaks. Cut the tart into slices and top with a dollop of whipped cream. Enjoy!