Separate eggs; let stand at room temperature for 30 minutes. Grease a 2-1/2-qt. souffle dish with 1 tablespoon butter; set aside.
Cut broccoli into florets; peel and chop stems. Place broccoli in a steamer basket; place in a Dutch oven over 1 in. of water. Bring to a boil; cover and steam for 4-6 minutes or until crisp-tender.
In a large saucepan, melt remaining butter. Stir in the flour, salt and pepper until smooth. Gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Add 1-1/2 cups cheese and mustard; stir until cheese is melted.
Remove from the heat. Stir a small amount of hot mixture into egg yolks; return all to the pan, stirring constantly. Stir in broccoli. Allow to cool slightly.
In a large bowl, beat egg whites until stiff peaks form. With a spatula, stir a fourth of the egg whites into broccoli mixture until no white streaks remain. Fold in remaining egg whites until combined. Transfer to prepared dish. Sprinkle with remaining cheese and bread crumbs.
Bake at 350° for 35-40 minutes or until the top is puffed and center appears set. Let stand for 15 minutes before serving.