Once the jars are lined, chill in the refrigerator for at least 15 minutes – 30 would be better if you have the time. This is also a great step to do a day ahead of time. While waiting for jars to chill, make your topping (also can do a day ahead of time). Put the flour, oats, brown sugar, cinnamon, and salt in a food processor and pulse several times to mix. Scatter the butter over the top. Pulse repeatedly until the mixture resembles fine crumbs. Empty the crumbs into a large bowl then rub them together between your fingers until you have large, buttery crumbs.