Cheddar Burgers With Balsamic Onions and Chipotle Ketchup

From Bon Apetit magazine July 2008. "The all-American cheeseburger get's a makeover, courtesy of grilled onions, a spicy-smoky ketchup, and an English muffin bun."

• Ready In: 30 mins
• Serves: 6
• Yields: 6 burgers

Ingredients

• Onions

• 1  lb red onion, cut crosswise into 1/3 to 1/2 inch rounds
•  olive oil
• 3⁄4 teaspoon kosher salt
• 1⁄2 teaspoon fresh coarse ground black pepper
• 2  tablespoons  balsamic vinegar

• Chipotle Ketchup

• 1  cup ketchup
• 1 1⁄2 teaspoons  chopped chipotle chilies, from canned chipotle chilies in adobo
• 2  tablespoons adobo sauce, from can
• 2  teaspoons  balsamic vinegar

• Burgers

• 2 1⁄4 lbs  80% lean ground beef (15%-20% fat)
•  coarse kosher salt
• 6  slices sharp cheddar cheese (thick slices)
• 6  large English muffins or 6  large  hamburger buns, split & grilled
• 6  slices tomatoes
• 2  cups fresh spinach leaves

Directions

1. Onions:
2. DO AHEAD; CAN BE MADE UP TO 3 DAYS AHEAD! Prepare grill to medium high heat. Arrange onion rounds on baking sheet. Brush with oil; sprinkle with 3/4 teaspoon coarse salt and pepper. Transfer onion rounds to grill rack; close cover. Cook until onions are tender, about 10 minutes. Transfer to a medium bowl. Toss with vinegar. Cover and chill.
3. Chipotle Ketchup:
4. DO AHEAD; CAN BE MADE UP TO 3 DAYS AHEAD!
5. Mix ketchup, chilies, adobo sauce, and 2 teaspoons vinegar in small bowl. Season with salt and more vinegar, if desired. Cover and chill.
6. Burgers:
7. Shape beef into 6 1/2 inch-thick patties. Sprinkle patties on both sides with coarse salt and pepper.
8. Prepare grill to medium high heat. 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 grilled English muffin or bun bottoms on plates; spread with ketchup. Top with burgers, tomatoes, spinach, and grilled muffin tops. Serve, passing remaining ketchup separately.