In a pot with a steamer basket or a microwave-safe bowl, steam the cauliflower florets until very soft, about 8–10 minutes.
In a skillet, melt the butter over medium heat. Add the diced onion and cook until softened, about 4–5 minutes. Add the garlic and sauté for another 30 seconds until fragrant.
In a blender or food processor, combine the steamed cauliflower, sautéed onion and garlic, heavy cream, nutmeg, and mozzarella cheese. Blend until smooth and creamy.
In the same skillet, add the spinach and cook for 2–3 minutes (until wilted if fresh or warmed through if frozen and drained).
Pour the cauliflower sauce over the spinach and stir until everything is well combined and heated through.
Taste and adjust seasoning with additional salt, pepper, or a touch more nutmeg if desired. Serve warm as a side dish.
Drain the spinach well: Thawed frozen spinach holds a lot of water. Squeeze it dry using a clean kitchen towel or paper towels to prevent a watery final dish.
Blend until fully smooth: The cauliflower sauce should be completely creamy with no visible texture. Blend for at least 1–2 minutes, scraping down the sides as needed.
Adjust sauce consistency: If the sauce seems too thick, add a splash of cream while blending. If it’s too thin, simmer the finished dish for a few extra minutes to reduce excess moisture.
Season at the end: Cauliflower absorbs salt differently than dairy-based sauces, so taste and adjust seasoning after everything is combined and heated through.
Best served warm: This dish has the creamiest texture when served shortly after cooking, once the sauce has fully coated the spinach.