Add the pancetta to a large skillet over medium heat. Cook, stirring often, until the fat is rendered, and the pancetta is crispy. Remove the pancetta with a slotted spoon and place it on a paper towel to drain excess grease.

Keep the skillet over medium heat. Stir in the onions, garlic and a pinch of salt and pepper. Cook until the onions soften, stirring often, about 5 minutes. Stir in the flour to create a roux.

Cook the flour and onion mixture for 2 to 3 minutes, stirring until it’s golden and fragrant and nutty. Slowly stream in the milk while whisking continuously. Continue to stir as you pour in the cream.

Cook for 5 to 6 minutes, stirring often, as the milk mixture thickens to a sauce that coats a spoon. Add the nutmeg.

Reduce the heat to low and stir in the parmesan cheese, 1 handful at a time. Stir until the cheese melts completely.

Remove the spinach from the bags and if there is any excess liquid or it seems wet, squeeze out the liquid with a dish towel. Add the spinach into the bechamel sauce and stir. Turn off the heat. Stir in ½ cup of the gruyere cheese. At this point, you can taste it if you wish to see if it needs any more salt and pepper.

Transfer the mixture to a 9x9 or 6x10 baking dish (or leave it in the skillet if it’s an oven-safe skillet). Sprinkle the remaining gruyere on top. Bake for 25 to 30 minutes, until bubbly and golden brown.

Serve immediately!

To make this ahead of time, either bake the entire thing off and reheat in a low oven, covered with foil, until warmed through. Or, make the mixture in the skillet and spread it in the baking dish. Store it in the fridge until 30 minutes before baking. Let sit out of the fridge for 30 minutes then bake as usual!