Boil or steam cauliflower in water until just tender, about 6 to 8 minutes. Drain and set aside.
Grease a 2-quart baking dish. Heat oven to 350°. In a medium saucepan, heat butter over medium heat. Add flour and stir until smooth and just bubbly. Add salt, pepper, and milk. Cook the sauce, stirring, until thickened and bubbly. Add the mustard and cheese and continue cooking, stirring, until cheese is melted.
Combine the cauliflower with sauce and spoon into the prepared baking dish.
Combine topping ingredients and sprinkle over the cauliflower mixture. Bake for 15 to 20 minutes, until the casserole is bubbly and topping is browned. Bake in lower half of oven so top doesn’t over brown.