Bacon-Cheddar Burgers with Caramelized Onions

Ingredients

• 1 leaf romaine leaves soft
• 0.5 pound bacon frozen sliced
• 4  hawaiian rolls 
• 4 slices sharp cheddar cheese extra-sharp thin
• 1.5 pounds ground beef 
• 3 tablespoons olive oil extra virgin extra-virgin
• 1.5 pounds onion sliced into 1/2-inch-thick rings (2 large)
• 4 servings salt 
• 4 servings pepper black freshly ground
• 3 tablespoons seasoned rice vinegar 
• 4 servings tomatoes sliced
• 2 tablespoons butter unsalted melted
• 1 tablespoon worcestershire sauce 

Equipment

• food processor
• bowl
• frying pan
• grill
• kitchen thermometer
• grill pan

Directions

1. Heat oil in a 12-inch heavy skillet over medium heat until it shimmers, then cook onions with 1/2 teaspoon salt and 1/4 teaspoon freshly ground black pepper, covered, stirring occasionally, until golden and softened, 15 to 20 minutes.
2. Reduce heat to medium-low, then add vinegar, scraping up the brown bits on bottom of the skillet, and cook, uncovered, stirring, until liquid evaporates, 1 to 2 minutes. Season to taste with salt and pepper.
3. Transfer to a small bowl and keep warm, covered.
4. Prepare grill for indirect-heat cooking over medium-hot charcoal (medium heat for gas; see grilling procedure).
5. Meanwhile, cut frozen bacon crosswise into eight sections and distribute evenly in bowl of a food processor, then pulse until finely chopped.
6. In a large bowl, distribute bacon evenly over beef and drizzle mixture with Worcestershire sauce.
7. Sprinkle evenly with 1 teaspoon salt and 1/2 teaspoon freshly ground black pepper.
8. Combine mixture as gently and thoroughly as possible with your fingers (be careful not to overmix). Form into 4 patties, about 4 inches in diameter and 1 inch thick.
9. Brush cut sides of buns with butter.
10. Oil grill rack, then sear patties directly over coals (or lit burner or burners), covered only if using a gas grill, moving to area of grill with no coals (or over shut-off burner) if flare-ups occur, until browned, 2 to 3 minutes on each side.
11. Move burgers to area with no coals (or over shut-off burner) and grill, covered, turning once, until they reach 155°F on instant-read thermometer, about 4 minutes total. Top with cheese to taste and heat, covered, until it melts and burgers register 160°F, about 1 minute.
12. Transfer burgers to a clean platter.
13. Grill buns, buttered side down, until grill marks appear, about 1 minute.
14. Assemble burgers with buns and accompaniments, topping cheese with caramelized onions.
15. •Onions can be made 3 days ahead and chilled, covered. Rewarm.•Patties can be formed 1 day ahead and chilled, covered. Bring to room temperature before cooking.•If you plan on doubling the recipe, be aware that you will have to cook the burgers in batches.•If you are unable to cook outdoors, burgers can be cooked in a heated grill pan over medium to medium-low heat, covered the whole time, until they reach 160°F, about 1 minute.