I use either a smaller rectangle 7 X 11 " glass baking dish or an eight inch square glass dish.
A 13 X 9 baking dish can be used to serve a larger crowd, but the ingredients need to be increased by 25 percent(ish.) This is a very flexible dish and can be tweaked with quantity or vegetable; all work!
Coat the glass dish with cooking spray.
Add one layer of sliced zucchini or sliced zucchini and squash.
Add the onions and peppers and cauliflower florets on top of the zucchini layer.
Pour the squash sauce over the vegetables, sprinkle with parmesan and crushed saltines.
Cheese alert! I am very generous with the cheese and liberally sprinkle the cheese on each layer so I use 1/2 cup in total. A 1/4 cup of cheese after the zucchini layer and then on top of all the vegetables before the crunchy topping is added.
The layered vegetable casserole is now ready for the oven.
Bake at 350 degrees for approximately 50 minutes. Zucchini and squash bake quicker than the cauliflower so check the dish for doneness by inserting a fork into the vegetables. The vegetables should be fork tender.
If additional cooking time is necessary, I cover the dish with foil as I do not want the crunchy saltine top to over brown. I baked the dish another 20 minutes.
Give the vegetables another fork test for tenderness. I prefer softer vegetables, but if a firmer vegetable is preferred then baking for 50 minutes total works well.