Wash the lettuce and pat it dry. Tear it into bite-sized pieces and put it in a bowl.
Slice the onions and toss them on top of the lettuce.
Fry the bacon until crisp.
Remove the bacon from the pan and crumble it.
In the same frying pan, pour in the sugar, vinegar and Dijon mustard, and whisk until smooth and syrupy.
If using the egg, add it now.
Immediately pour the sauce over the lettuce and serve promptly.
Notes
NOTE: Since I had precooked bacon, I simply warmed up the bacon in the pan with a little olive oil and then continued with the rest of the recipe as written above. Olive oil is better for you than bacon grease, so I suggest you try the recipe this way.