• 1/2 c. panko breadcrumbs (see Note)
Preheat oven to 400 degrees.
Combine milk, half-and-half and mustard in a measuring cup and set aside.
Heat a 12-inch ovenproof skillet over medium-high heat and add 4 tablespoons butter. When butter melts, add onion and sauté until it softens, about 2 minutes. Then add the spinach, 1/2 teaspoon salt and a few grinds of pepper. Sauté until spinach wilts, about 1 minute.
Whisk in flour and cook for 1 minute, stirring. Then whisk in milk mixture, stirring constantly. Be sure to scrape up any flour stuck on the bottom of the pan. Continue to stir until sauce thickens, about 2 minutes, and then stir in the cheese. Continue to stir until cheese melts.