HEAT oven to 350°F. COAT a lightly greased 2 to 2 1/2-quart soufflé dish evenly and completely with bread crumbs. MAKE a triple thick 4-inch wide band of aluminum foil, long enough to go around the dish and overlap by 2 inches. Lightly GREASE one side of the band and COAT with crumbs. WRAP around outside of dish, coated side facing in and extending at least 2 inches above the top. FASTEN with string. SET aside.
MIX flour, mustard and salt in medium saucepan; gradually WHISK in milk until smooth. COOK over medium-high heat, stirring constantly, until mixture thickens and boils. REMOVE from heat.
BEAT egg whites and cream of tartar in mixer bowl with whisk attachment on high speed just until whites are stiff but not dry, and no longer slip when bowl is tilted.
STIR egg yolks into cooked sauce until blended. ADD green onions and lemon juice; MIX well. Gently but thoroughly FOLD yolk mixture into whites until no streaks of white remain. Carefully POUR into prepared soufflé dish. For a “top hat”, HOLD metal spatula upright and MAKE a ring in top of soufflé mixture, 1 inch from side of dish and 1 inch deep, if desired.
BAKE in 350°F oven until soufflé is puffy, delicately browned and shakes slightly when oven rack is moved gently back and forth, 50 to 60 minutes. Quickly but gently REMOVE foil collar. SERVE immediately.
It’s easiest to separate eggs cleanly when they are refrigerator cold. However, egg whites whip up to greater volume when they’ve had a chance to warm up a bit, 20 to 30 minutes. Always begin by separating the eggs. Let the whites stand at room temperature while you prepare the sauce and other ingredients.
This recipe is an excellent source of protein and choline, and a good source of vitamin A and folate.