Preheat your oven to 350F. Coat a 7x11 baking dish (2 quart) with non-stick cooking spray and set aside.
In a large skillet, heat oil over medium heat. Lightly sauté sliced zucchini, chopped onion, garlic, Italian seasoning, and salt for 5 minutes or until vegetables start to soften.
Remove skillet from heat and stir in the cheddar cheese and 2 ½ cups of Monterey Jack cheese. Mix in mayonnaise until fully combined.
Add cheesy zucchini mixture to the prepared baking dish and sprinkle on the remaining cup of Monterey Jack cheese.
Cover with foil and bake for 25-30 minutes (Tip: spray the foil with non-stick spray to prevent the cheese from sticking).
Just prior to removing the casserole from the oven, melt the butter and combine it with the Panko breadcrumbs in a small bowl. Evenly sprinkle the casserole with the panko mixture.
Move your oven rack to the second highest level and broil, uncovered, until golden. Keep a close eye on it, it won't take long!
You can easily substitute the zucchini with yellow squash or include a mixture of the two.
We chose a combination of both cheddar and Monterey jack cheese. You can use old cheddar, mild, or sharp. Havarti, Gouda, or Muenster cheese are great substitutes for Monterey jack.
Store any leftovers in an air-tight container kept in the refrigerator for 3-4 days. Reheat in a 350F oven for 10-15 minutes or until warmed through.
You can leave the panko breadcrumbs and butter topping off for a lower carb alternative which clocks in at only 6g of carbs per serving.