Over medium, melt 3 tbs butter in a large pot. Add as much spinach as you can, adding more as it cooks down, if necessary. Cook until wilted but not soggy, 4-5 minutes. Remove from pot and set aside.
In that same pot, melt the remaining 5 tbs butter. Sprinkle in 5 tbs flour and stir to evenly combine. Stirring occasionally, cook until light golden brown, about 5 minutes.
Add chopped onion and garlic, stir, and cook until softened, 1-2 minutes.
Add milk, whisking while pouring to ensure the flour mix gets incorporated. Stirring occasionally, cook until thickened like a gravy, about 5 minutes.
Season cream with salt, pepper, and nutmeg. Add spinach and stir gently to combine. Serve immediately, topped with Parmesan, if desired.
