2) In a sauce pan, heat 355ml of the milk with the onion, bay leaf, and thyme to just below the boiling point. Set aside. In another small sauce pan, melt 15g of the butter over medium heat and then sift in the flour. Cook the mixture, stirring constantly with wooden spoon in a figure-8 motion, until it lightens in color, about 2 minutes. Remove from the heat and cool slightly.
3) Gradually whisk the milk mixture into the flour mixture. Bring to a boil over medium-high heat while whisking constantly. Reduce the heat to low, and simmer, whisking occasionally, until the sauce is thickened, about 10 minutes. Strain the sauce into the bowl of spinach. Stir to combine.
4) To serve, heat the creamed spinach in a saucepan over medium heat. Season with the 3/4 teaspoon salt, nutmeg, and pepper to taste; add the remaining 15g butter. (Adjust consistency with remaining milk, if necessary.) Serve.