Melt butter in a small saucepan over moderate heat and whisk in flour. Cook, whisking constantly, for two minutes. Whisk in milk in a stream, until smooth. Simmer, whisking, until thickened, about three minutes and reduced to 1-1/4 cups. Keep warm, covered, off heat. Fill a medium skillet halfway with water and bring to a boil. Break eggs into separate teacups and pour into water. Cover skillet and remove from heat. Let eggs sit for about 4 minutes until whites are set. Place bagels on serving plates and top each with 1/4 of spinach, using a slotted spoon top with a poached egg and 1/4 of sauce. Serve immediately.