- Melt butter in a saucepan, stir in flour, gradually add milk, stirring until thick and smooth.
- Add parsley, onion, tomatoes, cheese, salt, pepper and mustard. Stir until cheese is melted and sauce is smooth.
- Beat egg whites until stiff but not dry. Fold them into sauce.
- Pour into a buttered 1-quart souffle dish. Bake, uncovered, at 325 F for 50 minutes, or until souffle is lightly browned.
- Serve immediately.