Slice zucchini, sprinkle lightly with salt, let sit 10 minutes, then pat dry.
Heat olive oil in a skillet, add garlic, then cook spinach just until wilted; remove from heat.
Preheat oven to 375°F (190°C) and grease a 9×13-inch baking dish.
Whisk eggs, milk, salt, pepper, and nutmeg in a large bowl.
Stir in crumbled feta.
Add zucchini and spinach to the egg mixture and gently combine.
Pour mixture into the prepared baking dish and smooth the top.
Sprinkle Parmesan on top if using.
Bake uncovered for 35–40 minutes until set and lightly golden.
Broil 2–3 minutes if extra browning is desired.
Rest 10 minutes before slicing.