Combine all ingredients for filling in a large bowl. If the fruit seems too runny add another TB. of flour. (PS parboil peaches for easier pealing and cutting.)
Combine butter with oats and salt in a mixer fitted with paddle attachment.
Add sugar and finally the flour in stages. Do this on a relatively low speed so you don't wind up with a mess.
Use the butter wrappers to grease a large and shallow gratin dish. Pour in fruit. Carefully cover with topping so most of the fruit is covered.
Bake at 350 for about 50 minutes. Mixture should be bubbling nicely.