Cover cauliflower with salted water and bring to a boil. Boil for about 5 minutes or just until tender. If using frozen cauliflower, follow package directions. Drain, reserving 2/3 cup of the cooking liquid. Keep cauliflower warm while making sauce.
Heat 2 tablespoons butter in a medium saucepan over medium-low heat; stir in 2 tablespoons flour and cook for 1 to 2 minutes. Gradually add the 2/3 cup milk and the reserved 2/3 cup cooking liquid. Remove from heat and quickly stir in lightly beaten egg yolk and cheese. Return to heat and season with salt, pepper and mustard. Heat through until cheese is melted.
Butter a shallow baking dish. Arrange the 6 slices of toast in 1 layer (trim to fit if dish is an oval shape). Top toast layer with sliced ham, then top with cauliflower. Pour sauce evenly over the casserole then sprinkle with bread crumbs. Broil about 6 inches from heat (500 to 550° ) for 3 to 5 minutes, or until browned.
Calories per serving (with ham): 211