Soggy zucchini is the result of overcooking. Many casserole recipes call for par-cooking the zucchini before baking in the oven. This recipe does not require par-cooking because I think it's unnecessary and can easily lead to soggy zucchini. If your zucchini is cubed fairly thick (about 1 inch or larger cubes), and you're not cooking the casserole longer than directed, the zucchini will come out firm and perfectly cooked - not soggy.