Cook the bacon in a skillet over medium heat until crispy.
Remove the bacon and let it drain on a paper towel.
In a large bowl, combine the chopped romaine lettuce.
Add the sliced tomatoes and red onion to the bowl.
In a small bowl, mix the mayonnaise and Dijon mustard together.
Crumble the cooked bacon and add it to the salad.
Drizzle the mayonnaise mixture over the salad.
Toss the salad gently to combine all ingredients.
Season with salt and pepper to taste.
Serve immediately, garnished with croutons if desired.