Preheat oven to 350 degrees.

In a medium bowl, combine flour, oats, cinnamon, sugar, and butter until crumbly.

Press 1/2 of the flour and oat mixture into a 9 inch square pan.

Toss the blueberries with the lemon zest and cover the bottom of the pan.

Mix together cornstarch and sugar in a microwave safe bowl, gradually stir in water and heat until just boiling.

Pour water/cornstarch/sugar mixture over the blueberries.

Pour the rest of the flour/oat mixture over the top.

Cook for 45 minutes-1 hour.