Broccoli-Cheddar Puff

5 Tbsp. butter
6 Tbsp. all-purpose flour
1/2 tsp. salt
2 and 1/4 cups whole milk
8 ounces sharp Cheddar cheese, shredded (2 cups)
2 boxes (10 oz. each) frozen chopped broccoli, thawed and squeezed dry
7 large eggs, separated
1 and 1/2 cups coarse soft fresh bread crumbs (from 3 slices firm white bread)

In 4-quart saucepan, melt 4 Tbsp. butter over medium-low heat. Stir in flour, salt, until blended; cook 1 minute, stirring. Gradually stir in milk; cook until mixture boils and thickens, stirring frequently. Stir in Cheddar; cook just until melted. Remove from heat. Stir in broccoli.

In small bowl, with fork, lightly beat egg yolks. Stir in about 1/2 cup cheese sauce. Gradually pour egg-yolk mixture into cheese sauce, stirring rapidly to prevent curdling. Cool slightly.

Meanwhile, preheat oven to 325 degrees F. Grease shallow 3 and 1/2 quart ceramic casserole dish or 13x9-inch glass baking dish. In microwave-safe small bowl, heat remaining 1 Tbsp. butter in microwave oven on High 15 to 20 seconds or until melted, swirling bowl once. Add bread crumbs; stir until well combined.

In large bowl, with mixer at high speed, beat egg whites until stiff peaks form when beaters are lifted. With rubber spatula, gently fold one third of whites into cheese mixture. Fold cheese mixture gently back into remaining whites.

Pour mixture into prepared casserole. Sprinkle crumb mixture on top. Bake 40 minutes or until top is browned and knife inserted in center comes out clean. Serve immediately.