Preheat the oven to 350F. Grease a 9×13-inch pan or baking dish with butter or cooking spray. Set aside. (See note below if you wish to use a jelly roll pan instead of a 9x13 pan).
Combine the flour and sugar in a bowl. A stand mixer with a paddle attachment would be handy for this. Add the chilled butter cubes, salt and lemon zest, if using, and beat on medium speed until the butter is evenly distributed and the mixture is crumbly. Since I didn’t have a stand mixer with a paddle attachment, I used a pastry blender to incorporate the cold butter with the flour and sugar mixture. It was not a problem at all and the crumbly mixture was achieved.
Sprinkle the reserved crust mixture on top of the berry pie filling. Distribute well.
Bake for 60-70 minutes, or until the top is lightly browned. Mine needed about a little over an hour. Oven temperature and heating capacity vary so adjust accordingly. One of our readers noted that the berries bubbled over – because the fruits are fresh and juicy this may happen so you can prevent this by using a cookie sheet for any spill-over.
The original recipe which this was adapted from called for and recommended a 9x13 pan which I used (see photos). Please note that if you do that baking time would be much longer - about 1 hour, depending on the heat/temperature of your oven and this can vary so adjust accordingly. The bars would be thicker than usual because of the volume of the ingredients. But it would taste really good as this is a delicious recipe. The original recipe also called for about 6 cups of fresh berries, if you're using fresh berries and not the frozen ones.
Now, if you don't want thick bars - you may use a jelly roll pan like a 15-by-10-inch one or whichever jelly roll pan you have - a smaller size should be fine, too. Since this would be wider - use the full 6 cups of berries (if fresh) and 16 oz if the berries are frozen. Follow the exact same instructions but adjust the baking time. Check between 40-45 minutes to see if it's done.