Use to make Brucie chef Zahra Tangorra's Roasted Roma Tomato Lasagna.
Melt butter in a large saucepan over medium-high heat. Add flour and stir until absorbed, about 1 minute. Slowly whisk in milk, salt, and nutmeg. Bring to a boil and cook, stirring, until thickened, about 5 minutes. Remove from heat and let stand 15 minutes before using.