In a skillet cook bacon over moderate heat, stirring occasionally, until crisp. Reserving 1 tablespoon bacon fat in skillet, drain bacon on paper towels and crumble. Cut bread into enough 3 /4 - inch cubes to measure 1 cup. Heat fat over moderately high heat until hot but not smoking and saute bread cubes with salt to taste, stirring, until; golden brown. Transfer croutons to paper towels to drain and cool.
In a large bowl toss together onion, tomatoes, lettuce, half of bacon and croutons, salt and pepper to taste, and enough dressing to coat.
Divide salad between 2 plates and top with remaining croutons and bacon.