Ingredients:

1/2 cup (1 stick) unsalted butter, melted

1 cup all-purpose flour / plain flour

3/4 cup old-fashioned whole-rolled oats

1/2 cup + 1/3 cup granulated sugar / caster sugar

1/4 cup light brown sugar, packed

pinch salt, optional and to taste

12 ounces (2 cups) blueberries (fresh or frozen)

2 tablespoons lemon juice

2 teaspoons cornstarch

Directions:

1. Preheat oven to 350F. Line an 8-inch square pan with aluminum foil and spray with cooking spray. Set aside. Lining the pan helps with cleanup and is recommend.

2. 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, and using a spatula or your fingers, hard-pack the mixture to create an even, smooth, flat crust. Set aside.

3. In a large mixing bowl. Add all ingredients and toss to combine. If sugar hasn't dissolved fully that's okay because it liquefies while baking. Evenly distribute blueberry mixture over the crust. Evenly sprinkle with the reserved heaping 1 cup crumble topping mixture. Bake for about 45 minutes, or until edges are set and center has just set. Crumble topping should appear set and very pale golden.

4. Watch your bars, not the clock, when evaluating doneness. Place pan on a wire rack and allow bars to cool for at least 30 minutes before slicing and serving. If you try to slice bars before they've cooled completely, they'll be extremely messy and could fall apart.

5. Bars will keep in airtight container at room temperature for up to 1 week, in the fridge for up to 10 days, or in the freezer for up to 6 months.

Bon Appetit!