Preheat oven to 350°F.

Place egg yolks, milk, flour and salt in blender. Blend until smooth. Pour into bowl and let stand.

Melt butter in small skillet over medium heat. Add onion; cook until tender.

Pat canned Chile's dry with paper towels. Slit each chile lengthwise and carefully remove seeds. Place one strip cheese and 1 tablespoon onion in each chile; reshape chile to cover cheese.

Place in single layer in 13x9-inch baking dish.

Beat egg whites until soft peaks form; fold into yolk mixture. Pour entire mixture over casserole.

Bake uncovered 20 to 25 minutes or until topping is puffed and knife inserted in center comes out clean. Broil 4 inches below heat 30 seconds or until topping is golden brown. Serve with condiments.