Blanch the spinach in boiling water for only one minute, then transfer it to a cold water bath to stop cooking. Gently squeeze out any remaining water from the leaves and set aside.
Cook the bacon in a large skillet over medium heat until it turns crispy. Use a slotted spoon to transfer it onto a plate lined with paper towels to absorb excess oil. Keep it aside.
Remove most of the bacon grease from the pan, leaving about 1-2 teaspoons for extra flavor (adjust to your preference). In the same pan, melt the butter, then add the onion and cook until it turns translucent (approximately 2 minutes), scraping up any browned bits left behind by the bacon.
Whisk in the flour and cook for a couple of minutes until light golden.
Gradually add the half and half while whisking constantly. Then, mix in the Boursin until it's well combined. Continue to whisk for about 2 minutes, or until the white sauce thickens.
Season with nutmeg, cayenne (if using), salt and pepper. If the sauce is too thick, add another 1/4 cup of half & half, whisking until it reaches your preferred thickness.
Add the cooked spinach to the boursin sauce, stirring gently to combine. Sprinkle some parmesan cheese, add the chopped bacon and give everything a good mix to combine all of the flavors together.
Serve immediately and enjoy!