Set a wire rack over a foil-lined baking sheet. Add the bacon slices to the wire rack and transfer to the oven. Roast for 8-10 minutes, until crisp. Drain on paper towels and when cool, chop into pieces.
Trim the ends off of each head of Romaine and slice the heads in half.
Place a Romaine half on each of 4 chilled salad plates and pour 2 or 3 tablespoons of blue cheese dressing on each serving. Divide the bacon, diced onion, tomatoes and crumbled blue cheese equally to each salad.