See bottom for adaptations for other fruits. Don't let the picture deceive you - this makes a lot of topping; my blackberries just bubbled over a lot of it.
Blackberry (or other fruit) Crisp
3/4 c. flour
1 c. rolled oats
1 c. brown sugar
1 t. cinnamon
1/2 c. softened butter
4 c. frozen blackberries
1/2 c. sugar
1/4 c. flour
juice of 1/2 lemon
In a large bowl, combine frozen blackberries with sugar. Let macerate on the counter for about 1/2 hour (or up to a few hours), until blackberries begin to thaw slightly and combine with the sugar. Add flour and lemon juice. Mix to combine.
Combine ingredients for crumb topping, cutting in butter with a fork until the mixture is crumbly. Pour the fruit filling into an 8x8 pan and top with the crumb topping. Bake at 375 degrees for 30-35 minutes, until topping is browned and juices are bubbling. Serve warm with ice cream.
If using apples, reduce sugar in filling to 1/4 cup, omit flour, and add 1 t. cinnamon. Bake for 40-45 minutes until tender. Mixed berries would also be delicious or cherries, apricots, or anything you like!