Growing up (and beyond) my mom would make this in the summer for dinner with out a recipe. It's very flexible and forgiving, depending on what you have, what's in season, and what you like, but it's always delicious. —kedivine
Grease a 9.5 inch pie plate lightly with extra virgin olive oil and pre-heat oven to 375 degrees.
Layer the zucchini slices around in a circular motion, overlapping just slightly to make one layer. Salt and Pepper EACH layer with a heavy hand! If there are more slices (about a standard mandolin slice thickness), add an additional layer and season again.
Next, make a layer of tomato slices and season. Follow this with the mozzarella - make a layer of thinly sliced mozzarella so that it is mostly covering the entire surface, and spread out the sliced red onion on top - season again. I like to sprinkle about half of the basil in this layer as well.
Now make your last layer (or two) of the yellow squash. Season again, and top with just a little bit of the panko, a little bit of the basil, and cover with tin foil. Place in the oven for about 30-45 minutes - checking a few times and pouring off some of the liquid if necessary.
Remove the foil lid, top with reserved panko, parmesan and reserved basil and cook for another 20-30 minutes, until the top is nicely browned and crisped. Sometimes I am impatient and crank the oven up to 400 at this step, though the effects may be all in my head. Continue to check and pour off extra liquid if necessary.