In a nonstick skillet, melt butter over medium-high heat and add the onion. Sauté until softened and translucent, about 2-3 minutes. Then add garlic and cook for a few seconds until fragrant, stirring constantly.
Add the vegan cream, milk, nutritional yeast, salt, pepper, and nutmeg. Stir until well combined. Bring to a low simmer and cook for a few minutes. The sauce should thicken slightly as it cooks.
Add the spinach into cream sauce until evenly combined, cooking for an additional minute or so until the spinach is wilted all the way through.
Taste and adjust seasoning, if necessary. (If the spinach seems a little watery, cook over medium-low heat for a few minutes until any excess liquid evaporates.)