In a large nonstick skillet, melt butter over medium-high heat. Add onion and sauté until softened and translucent, stirring frequently, about 2-3 minutes.
Add garlic and cook for 30 seconds until fragrant. Add cream cheese and cook, stirring, until melted.
Add heavy cream, salt, pepper, nutmeg, and cayenne; cook for 2 minutes.
Add mozzarella cheese and parmesan cheese. Stir until combined and melted.
Fold spinach into cream sauce until evenly combined, cooking for an additional minute or so to warm through.
Taste and adjust seasoning, if necessary. (If the spinach seems a little watery, cook over medium-low heat until any excess liquid evaporates.)
Serve immediately while the sauce is still shiny and cheese is gooey.
