Preheat the grill to high heat.
Heat 1 tablespoon of the olive oil in a saute pan over high heat. Add the onions and bacon and cook until the bacon is crispy. To the same pan add the balsamic vinegar and 1 tablespoon of the olive oil and stir to combine. Remove from heat and set aside.
Brush the romaine lettuce with the remaining 2 tablespoons of olive oil, place on the grill cut side down, and quickly sear.
Serve the lettuce, cut side up, and drizzle the balsamic dressing over the lettuce. Sprinkle with blue cheese and garnish with cracked black pepper.