Add the cherries, sugar, lemon juice, cornstarch and cinnamon to a very large stock pot. Toss with a spoon until the cherries are evenly covered.
Heat the mixture over low heat stirring occasionally. The cherries will first start to release their juices, and then the mixture will start to thicken.
After the juices have thickened (about 10-15 minutes), remove from the heat.
Pour the mixture into a 9x13 inch glass or ceramic pan.
In a medium bowl stir together the flour, oats, and sugar.
Stir in the melted butter. The mixture should look like wet sand.
Crumble the oatmeal topping over top of the cherries.
Bake in the oven for 25-30 minutes, or until you see the top starting to brown and cherries bubbling through. If the top is getting too brown, tent a piece of aluminum foil over top and continue baking.
*If using frozen berries, they'll need to boil for 5-10 minutes longer. **Large rolled oats or large flake oats work too. Do not use instant or steel-cut oats.Recipe can be halved and made in an 8x8 or 9x9 inch ceramic or glass pan.Store leftovers covered in the fridge.