Preheat the oven to 350°F.

Slice the vegetables: Using a mandolin or sharp knife, slice the zucchini and yellow squash into ¼-inch rounds. Thinly slice the onion.

Layer the casserole: In a 9x13 baking dish, layer 1/3 of the squash and zucchini, followed by 1/2 of the onions.

Season with salt, pepper, mustard powder, and nutmeg (if using).

Sprinkle half of the shredded cheese over top.

Repeat the layer with another ⅓ of the squash, zucchini, the remaining onions and seasonings, then the remaining cheese.

Finish with the last layer of squash, zucchini, and seasonings.

Make the topping: Crush the Ritz crackers in a zip-top bag using a rolling pin or meat mallet.

Add the grated Parmesan cheese and melted butter, then mix until the crackers are evenly coated.

Spread the mixture evenly over the top of the casserole.

Cover tightly with foil and bake for 45 minutes.

Uncover and bake an additional 15 minutes, until the vegetables are fork-tender (but not mushy) and the topping is golden brown.

Serve warm and enjoy that perfect combination of cheesy, buttery, and savory goodness!