For the pan pictured (about the size of a standard Corning Ware casserole) I used:
1/2 lb. salted butter
1 cup brown sugar (packed down)
2 cups flour
Place a little of the flour and sugar at the bottom of the dish, add your fruit, top it with the rest of the flour and sugar mixed in with the butter. (Note: If you use unsalted butter add a dash of salt.)
BAKE: 375 degrees for 45 minutes.
My rhubarb went in chopped up. Fresh. You can use canned apples (found in the canned fruit section at your grocers) and just add the crumble topping and get a great homemade dessert effect. I advocate fresh everything, but with apples, you'll have a little more work. Peeling and slicing, mixing them first in a bowl with some sugar and squeeze of lemon. Fresh peaches need peeling and rhubarb only chopping.