Add all cherry filling ingredients to small sauce pot (2.5 c frozen cherries, 3 T water, ¼ c white sugar, 1 T corn starch, 1 T lemon juice, pinch of salt). Cook, stirring occasionally until sauce thickens. This should take between 5-8 minuets. Break up cherries with a spoon or your rubber scraper if you desire smaller chunks of cherries. Add 1-2 T more lemon juice depending on how sweet the filling is. Place in refrigerator to cool.
Preheat oven to 350 degrees. While cherry filling is cooling, prepare oatmeal mixture. This will be used for the base and for the topping. Add ¾ c whole wheat flour, 1 c old fashioned rolled oats, ½ c dark brown sugar, ½ teaspoon baking powder and ½ teaspoon salt to large mixing bowl. Mix together. Add ½ cup chopped cold butter. Cut into flower/oat mixture with two knives or with a pastry cutter until butter is pea sized. (watch video to learn how to cut butter into flour mixture.
Prepare bars by placing parchment paper in an 8x8 metal baking pan, then add ½ of the flour/oatmeal mixture into baking pan. Press down with a measuring cup or your clean hands to form a pressed crust with an even thickness. Pour cooled cherry mixture over base, spread over crust. Sprinkle remaining flour/oatmeal mixture to make crumbly topping. Do not press down topping. Bake for 20-30 minutes, or until cherry filling is bubbling and topping is golden brown. Remove and cool before serving. Eat as is or serve with vanilla ice cream or yogurt. Refrigerate leftovers for 3-5 days.