Preheat oven to 350°F. Set aside a 9-inch pie pan.
Toss together 3 cups oats and maple syrup. Spread evenly onto a baking sheet and toast for 15 minutes, stirring halfway through cooking time. Remove from oven and let cool.
Place toasted oats in the bowl of a food processor and add hazelnut meal, brown sugar, cinnamon and salt. Process until the oats break down and form a uniform mixture. Remove 1 cup of mix and set it aside.
With the food processor running, stream melted butter into the mixture until well mixed.
Press the mixture into the bottom and sides of the pie pan to make a crust then bake for 10 minutes. Remove from the oven and let cool.
Cherry Filling
Place cherries and sugar in a pot and heat over medium heat until the sugar dissolves. Bring the mixture to a simmer.
While the cherries cook, combine either the reserved cherry juice or water with the lemon juice. Add the cornstarch and mix to form a slurry. When the cherries have begun to simmer, add the cornstarch slurry and mix, stirring frequently, until the mixture thickens and the liquid is not longer cloudy from the cornstarch, 3-5 minutes.
Pour the filling into the prepared crust.
Hazelnut Crumb Topping
In a medium bowl, combine the reserved crust mix and 1 cup oats. Add the cold, cubed butter and mix until the mixture is chunky. Sprinkle evenly over the top of the cherry filling.
Bake until the topping is lightly browned and the cherry filling is bubbling, 40-50 minutes. Let cool for 1 hour before serving.
