For the blueberry sauce: Combine the blueberries, granulated sugar and 1/2 cup water in a saucepan. Bring to a gentle boil and cook until the berries begin to burst, about 5 minutes. Stir together the cornstarch and 1 tablespoon water in a small bowl, then whisk into the berries. Bring to a boil to thicken the sauce, mashing with a wooden spoon or whisk. Pour into a container and let cool, then refrigerate until completely chilled, about 2 hours.
For the crumble topping: Meanwhile, preheat the oven to 350 degrees F.
Combine the flour, oats, pecans, granulated sugar, brown sugar and salt in a food processor and pulse to combine. Scatter in the butter pieces and pulse until the mixture resembles coarse crumbs. Add the heavy cream and egg yolks and pulse a few times just to combine.
Pat the mixture into an 8-by-10-inch rectangle on a parchment-lined baking sheet. Bake until crisp with browned edges, 18 to 20 minutes. Transfer the baking sheet to a rack and let cool completely, at least 15 minutes, then crumble.
For the sundaes: To serve, layer the ice cream and blueberry sauce in tall glasses. Top with whipped cream and sprinkle with the crumble topping.