Preheat the oven to 375 degrees F. Butter an 8 by 8-inch baking dish. Melt
the butter in a large skillet over medium-high heat. Add the onion and a pinch of salt. Saute for 3 minutes, then add the garlic and red chili flakes and saute for 2 more minutes. Add the heavy cream
, Parmesan, and nutmeg. Heat until the cream is just beginning to bubble, stirring often. Turn the burner off and stir in sour cream and spinach
. Adjust the seasoning with salt and pepper, to taste. Pour into the prepared baking dish
and sprinkle the top evenly with fried onions
. Bake until golden and bubbly, 15 to 20 minutes.