Preheat the oven to 400°F.
In a casserole dish, pie dish or cast-iron skillet, add the blueberries, sugar, cornstarch and cinnamon and mix until the blueberries are fully coated.
6 cups fresh or frozen blueberries, 3 pints, 1/3 cup granulated sugar, 1 ½ Tbsp cornstarch, ½ tsp cinnamon
In a separate bowl, combine the flour, oats, brown sugar, cinnamon, pecans, and salt.
2/3 cup all-purpose flour, 1 cup old-fashioned oats, 1/3 cup brown sugar, ½ tsp ground cinnamon, 1/3 cup pecans or walnuts or almonds, chopped , ¼ tsp salt
Add in the cold cubed butter and with your hands or a dough cutter mix until the mixture is crumbly. (Don’t overmix — you still want some chunks of butter. It should be crumbly, not smooth.)
1/2 cup unsalted butter , cold and cubed
Add the crisp topping over the blueberries, sprinkling evenly over all the fruit.
Bake at 400°F for 35-40 minutes, until the topping is golden brown and juices are bubbling around the edges. Remove it from the oven and cool it for 5 minutes.
Serve warm with vanilla ice cream, yogurt, or plain.