Heat the oven to 375°F (190°C) and grease a 2-quart casserole dish or approximately 9-inch square baking dish. Set the prepared dish aside while you cook the vegetables.

Melt the butter in a skillet over medium heat. Add the onion and cook for 5–7 minutes, stirring occasionally, until softened but not deeply browned.

Add the minced garlic and cook for 30–60 seconds, just until fragrant. Stir in the spinach and cook fresh spinach until wilted, or heat thawed frozen spinach until warmed through.

Remove the spinach mixture from the heat and press or squeeze out excess moisture thoroughly. Spread the drained spinach and onion mixture evenly in the prepared baking dish.

Whisk the eggs, heavy cream, remaining salt, black pepper, and optional nutmeg in a large bowl until blended. Stir in half of the Parmesan cheese.

Pour the egg mixture evenly over the spinach. Sprinkle the remaining Parmesan over the top so it forms a lightly browned, savory crust during baking.

Bake for 35–45 minutes, checking toward the shorter end of the range. Remove the casserole when the center is set but still slightly soft and the top is lightly browned.

Rest the casserole for 5–10 minutes before slicing. The eggs will finish setting from the residual heat while keeping their creamy texture.