This recipe is based on Blueberry Crumble Bars by magicallydelicious.

Crust:

• 1 1/2 cups all-purpose flour
• 1 1/2 cups quick-cooking oats
• a bit less than 1 cup of sugar
• 1/2 teaspoon ground cinammon
• 1/2 teaspoon baking soda
• 1 cup cold butter

Filling:

• 2 1/2 cups of blueberries (want a high, consistent density of blueberries throughout the pan)
• 1/2 cup sugar
• 2 tablespoons corn starch
• 3 tablespoons lemon juice

Mix together the crust ingredients in a big bowl. Use two knives and a pastry blender to cut the cold butter into the dry ingredients until the butter pieces are small.

Set aside a little more than two cups of the mixture for use on the top.

Press the remaining mixture into a 9x13" greased pan. Press using the bottom of a spatula to make it flat.

Mix the filling ingredients in a medium-sized pot.

Heat the filling over medium heat while stirring constantly.

Once it starts boiling, some of the blueberries will leak, turning the fluid purple.

Turn off the heat once the liquid starts to thicken up. You want it to be thicker than water but still easy to spread, like jelly. (The longer you boil it, the thicker it gets.)

Spread the filling over the pan. Make sure there are no gaps: spread the liquid part of the filling over any gaps or place fresh blueberries in the gaps.

Spread the remaining crust mixture on top of the filling.

Add another dash of cinnamon to the top if you want.

Bake at 375° F for 25 to 45 minutes. You can maybe tell if they are done by seeing the top start to brown. It's normal to see some boiling at the edges of the pan.

Cut them any time after they are cool. They should remain soft and easy to cut for many days. The bottom crust should be very soft, but still hold strong enough to hold reasonably-sized pieces together.

These bars last a long time, I tried some 4 days after they were baked, and they were still soft and delicious.