Place the butter, chopped onions, and garlic in a skillet. Set over medium heat and sauté to soften, about 3-5 minutes. Stir in the flour and stir well to break up the clumps.
Scoop the onion mixture into the crock of a large slow cooker. Add the frozen spinach, milk, salt, nutmeg, white pepper and cayenne pepper. Cover and turn on low.
Slow cook for 8-10 hours until the mixture simmers and thickens. Stir every few hours. When ready to serve, stir in the Parmesan cheese. Serve warm.
Video
Notes
Stovetop Instructions: Cook the onions and garlic in butter as directed above, but do so in a large deep saute pan. Add additional ingredients to the saute pan (Step 2) and bring to a simmer. Simmer for 15-20 minutes, stirring occasionally, to thicken. Stir in the Parmesan cheese and enjoy!