Place 5 ounces spinach leaves in a colander, rinse them under running water and drain. Chop the leafy greens roughly using a knife.
Peel and finely chop 1 medium onion. Peel 1 clove garlic and press it.
Heat a frying pan over medium heat and melt in 1 ½ Tablespoons unsalted butter. Add the onion and fry until golden brown. Don't forget to stir so it doesn't burn! Stir in the pressed garlic and fry for another minute or so, but no longer, or the garlic will become bitter.
Pour 1 ¾ cups milk into a glass or bowl and add 1 ½ Tablespoons all-purpose flour, whisking well so there are no lumps of flour in the milk.
Immediately add the mixture to the pan with the sautéed onions and garlic. Stir well, then reduce the heat to low. Season with 1 teaspoon salt, ¼ teaspoon ground black pepper, and ¼ teaspoon nutmeg.
Stir in the spinach leaves, cover the pan with a lid, and let it simmer for ten minutes.
Transfer the cooked sauce to a blender, add 1 ½ Tablespoons apple cider vinegar, and blend for about a minute until smooth and creamy.
If needed, adjust the taste with additional salt, pepper, or a bit more vinegar. Your sauce is now ready to serve!