Heat a grill or grill pan and place the lettuce wedges on a platter.
Prepare the vinaigrette by briskly whisking together the Dijon, vinegar, and garlic. Drizzle in the olive oil while whisking to create an emulsion. Add the sugar and blue cheese, and whisk gently to mix. Season with salt and pepper to taste. Lightly oil the tomato slices, and season with salt and pepper. Grill until slightly charred on the outside and then transfer to the platter with the lettuce wedges. Pour the vinaigrette over the lettuce and tomato and top with bacon and red onion.