Preheat oven to 350 and grease a 2- or 3-quart casserole dish (I use a 9"x13")
Melt 4 Tbsp. butter in saucepan until sizzling and add zucchini, onion, salt, pepper, Italian seasoning, and water. Steam zucchini until tender, stirring occasionally.
Meanwhile, beat eggs, then stir in milk, horseradish, and grated cheese. (Or toss all in the blender and pulse until eggs are well-mixed and cheese is broken up.)
Add steamed zucchini directly to the greased casserole dish, then pour egg mixture over top. Gently stir ingredients to evenly distribute zucchini chunks.
Melt 3 Tbsp. butter and stir into cracker crumbs. (Or dice cold butter and dot on top of the cracker crumbs in next step.)
Sprinkle cracker crumb topping over the casserole, with an extra dash of Italian seasoning, if desired.
Bake for 30 minutes or until egg is set and top is golden.