Thaw the spinach in your microwave and work in batches, squeezing out as much water as possible. I place 1/3 of the thawed spinach into a clean kitchen towel (not a fancy one that’s for sure!) at a time and twist and squeeze with all that I have.
In a deep sided skillet, add olive oil, onion and garlic with a pinch of kosher salt. Stir and cook over medium to medium-low heat, until translucent and golden in some spots. This should take about 8 minutes.
Then add in the condensed cream of chicken soup and pour in the milk. Season with kosher salt, black pepper and nutmeg. I always eyeball the nutmeg. Stir until the soup has thinned out.
Remove off of the heat and add in the prepped chopped spinach and parmesan cheese. Stir well to combine.
Transfer the spinach mixture to a lightly greased, 2-quart casserole dish. Sprinkle with gruyere before baking on the middle rack of your preheated 400° oven for 30 to 40 minutes. Cool slightly before serving.