Adjust oven rack to the lower third of the oven. Preheat the oven to 350 °F (175 °C) and get out a 1-quart (1-liter) casserole dish or an 8 × 8-inch (20 × 20 cm) baking dish.
In a medium mixing bowl, mix together all of the topping ingredients. It might be quite wet.
2/3 cup (83 grams) flour, 1 1/4 cups (115 grams) rolled oats, 2 teaspoons ground cinnamon, 2/3 cup (132 grams) brown sugar, 2/3 cup (149 grams) unsalted butter, 1/4 teaspoon salt
Prepare the fruit filling. In a large mixing bowl, add the apples and then the remaining ingredients, except for the blueberries.
4 1/2 cups (495 grams) apple chunks, 1 1/2 tablespoons (12 grams) all-purpose flour, 1/4 cup (50 grams) brown sugar, 2 teaspoons lemon juice, 1 teaspoon ground cinnamon, 1 teaspoon vanilla extract
Stir until well combined, and then carefully stir in the blueberries.
1 1/2 cups (215 grams) fresh blueberries
Spoon the fruit mixture into the pan and distribute the topping evenly over the top. It'll be a thick layer of topping.
Place the crisp on a baking sheet to catch spills and bake for 35-45 minutes or until the topping is firm and the edges are bubbly. Let cool for 2-3 hours, which will give the juice some time to thicken a little, for the fruit to soften more and become more jammy.
Let cool completely before covering and refrigerating for up to 4 days.