1. Preheat oven to 350°F. Grease 8-inch quiche dish or pie plate. In 1-quart saucepan over medium heat, melt butter or margarine; stir in flour, dry mustard, and salt until smooth; cook 1 minute. Gradually stir in milk; cook, stirring constantly, until mixture boils and thickens. Remove saucepan from heat.
2. In small bowl, beat egg yolks slightly. Into egg yolks, beat small amount hot milk mixture. Gradually stir egg-yolk mixture into milk mixture in pan until blended, stirring rapidly to prevent lumping.
3. Meanwhile, in small bowl, with mixer at high speed, beat egg whites until stiff peaks form. With rubber spatula or wire whisk, gently fold yolk mixture and tuna into beaten egg whites. Pour mixture into prepared dish. Bake 20 to 25 minutes, until puffy and golden and knife inserted in center comes out clean.