In a shallow bowl beat together eggs, sour cream, dill, and mustard. Dip bread into egg mixture, allowing bread to soak in egg mixture about 15 seconds on each side.
In a skillet or on a griddle cook bread in hot margarine over medium heat for 2 to 3 minutes or until bottom is golden brown. Turn slices over. Place a cheese slice on top of bread. Top with a tomato slice; sprinkle with chives. Season with salt and pepper, if desired. Cook 2 to 3 minutes more or until bottom is golden brown, adding more margarine as needed. Serve warm. Makes 6 slices.