Preheat oven to 350 degrees. Butter a 12-inch cast iron skillet or 9x13 baking dish.
Measure sugar into a mixing bowl. Add the lemon zest, then use a fork to incorporate them together. Add the corn starch and mix well, then add the blueberries and combine well. Place the blueberries in your skillet or baking dish.
Combine flour, brown sugar, and oats in a large mixing bowl. Cut the butter in until the mixture is crumbly. It's easiest to do when the butter isn't right out of the refrigerator, but when it's had a few minutes on the counter--but not at room temperature or softened.
Cover the blueberries with the crumbly topping, then bake for 45 minutes or until golden brown and bubbly.