Cheddar Burgers with Balsamic Onions and Chipotle Ketchup

Ingredients

• 2 teaspoons balsamic vinegar ()
• 1.5 teaspoons chipotle chiles from chipotles in adobo* plus 2 tablespoons adobo sauce from can canned chopped
• 6 servings coarse kosher salt 
• 2.3 pounds ground beef 15% 20% ( to fat)
• 0.5 teaspoon coarsely ground pepper black
• 6 large hamburger buns split english
• 1 cup catsup 
• 6 servings olive oil 
• 1 pound onions red
• 6 slices sharp cheddar cheese thick
• 2 cups spinach leaves fresh
• 6 slices tomatoes 

Equipment

• bowl
• baking sheet
• grill

Directions

1. Prepare barbecue (medium-highheat): Arrange onion rounds on baking sheet.
2. Brush with oil; sprinkle with 3/4 teaspoon coarse salt and pepper.
3. Transfer onion rounds (still intact) to grill rack; close cover. Cook until grill marks appear, about 4 minutes per side. Reduce heat or move onions to cooler part of grill. Close cover; cook until onions are tender, about 10 minutes.
4. Transfer to medium bowl. Toss with vinegar. DO AHEAD: Can be made 3 days ahead. Cover; chill.
5. Mix ketchup, chiles, adobo sauce, and 2 teaspoons vinegar in small bowl. Season with salt and more vinegar, if desired. do a h e a d Can be made 3 days ahead. Cover and chill.
6. Shape beef into six 1/2-inch thick patties.
7. Sprinkle patties on both sides with coarse salt and pepper. Prepare barbecue (medium-high heat).
8. Place burgers on grill. Close cover; cook burgers until bottoms start to darken and juices rise to surface, about 3 minutes. Turn burgers; cook to desired doneness, about 3 minutes longer for medium-rare. Top with onions and cheese. Close cover; cook until cheese melts.
9. Place muffin bottoms on plates; spread with ketchup. Top with burgers, tomatoes, if desired, spinach, and muffin tops.
10. Serve, passing remaining ketchup separately.
11. *Dried, smoked jalapeños in a spicy tomato sauce called adobo; available at some supermarkets and at specialty foods stores and Latin markets.