Melt the butter in a large pot over medium heat.
Once melted, sprinkle in the flour gradually while whisking until you get a smooth mixture.
Continue to whisk the butter-flour mixture for about 5 minutes until it turns a light golden brown.
This step is critical to avoid that raw flour taste.
Slowly add the heavy cream while whisking the mixture.
Keep stirring for a couple of minutes until it thickens slightly.
Add the chopped onion and minced garlic.
Cook for another minute to allow the flavors to blend well.
Finally, add the spinach, salt, and pepper.
Stir everything together and cook on low heat until the spinach is completely wilted and coated in the sauce.