Heat oven to 350°F. Spray and lightly flour 1-quart souffle dish. Cut 24x12-inch sheet of foil; fold into thirds to form 24x4-inch rectangle. Place foil around top of dish with 1 inch inside dish and 3 inches above rim; secure with masking tape on outside of dish. Spray inside of foil.
In small bowl, beat egg yolks slightly. Set aside. Melt butter in medium skillet over medium heat. Add flour and salt; cook and stir until smooth. Gradually add milk, cooking until mixture boils and thickens, stirring constantly. Remove from heat.
Gradually add about 1/4 cup hot mixture to egg yolks; mix well. Return saucepan to heat; gradually add yolk mixture to milk mixture in saucepan, cooking and stirring until well blended. Stir in cheese until melted. Stir in tomatoes and chives. Remove from heat.
In medium bowl, beat egg whites until stiff. Fold into cheese mixture. Carefully pour into sprayed and floured dish.
Bake at 350°F. for 35 to 40 minutes or until souffle is puffed and top is golden brown. Serve immediately.