Crust and Crumble Topping
1/2 cup (1 stick) unsalted butter, melted
1 cup all-purpose flour
3/4 cup old-fashioned whole-rolled oats (don’t use quick cook or instant, they’re finer and behave like flour and will make mixture too dry)
1/2 cup granulated sugar
1/4 cup light brown sugar, packed
pinch salt, optional and to taste

Blueberry Layer
12 ounces (2 cups) blueberries (fresh or frozen; I used frozen and didn’t thaw)
1/3 cup granulated sugar
2 tablespoons lemon juice
2 teaspoons cornstarch

Preheat oven to 350F. Line an 8-inch square pan with aluminum foil and spray with cooking spray; set aside. Crust and Crumble Topping – In a large, microwave-safe bowl melt the butter, about 1 minute on high power. Add the flour, oats, sugars, optional salt, and stir to combine. Mixture will be dry and sandy with some larger, well-formed crumble pieces. Set 1 heaping cup mixture aside to be sprinkled on later as crumble topping. Transfer remaining mixture to prepared pan, hard-pack the mixture to create an even, smooth, flat crust; set aside. Blueberry Layer – In a large mixing bowl, add all ingredients and toss to combine. Evenly distribute blueberry mixture over the crust. Evenly sprinkle with the reserved heaping 1 cup crumble topping mixture. Before sprinkling, I squeeze the mixture in my palm to encourage bigger crumble pieces to form. Bake for about 55 minutes, or until edges are set and center has just set. Crumble topping should appear set and very pale golden. Place pan on a wire rack and allow bars to cool for at least 30 minutes before slicing and serving.

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