Heat oven to 400°F. Spray 9-inch pie plate with cooking spray. In 10-inch skillet, melt butter over low heat. Add onion; cook, stirring occasionally, until tender. Sprinkle tuna, 1/2 cup of the cheese and the onion in pie plate.
In medium bowl, stir remaining ingredients except tomato with wire whisk or fork until blended. Pour into pie plate.
Bake 25 to 30 minutes or until knife inserted in center comes out clean. Top with tomato slices and remaining 1/2 cup cheese. Bake 3 to 5 minutes longer or until cheese is melted. Let stand 5 minutes before serving.