Cheddar Burgers With Balsamic Onions And Chipotle Ketchup

• 1 pound red onions, cut crosswise into 1/3- to 1/2-inch-thick rounds
• Olive oil
• 3/4 teaspoon coarse kosher salt
• 1/2 teaspoon coarsely ground black pepper
• 2 tablespoons balsamic vinegar
• 1 cup ketchup
• 1 1/2 teaspoons chopped chipotle chiles from canned chipotles in adobo* plus 2 tablespoons adobo sauce from can
• 2 teaspoons (or more) balsamic vinegar
• 2 1/4 pounds ground beef (15% to 20% fat)
• Coarse kosher salt
• 6 thick slices sharp cheddar cheese
• 6 large English muffins or hamburger buns, split, cut sides grilled
• 6 tomato slices (optional)
• 2 cups fresh spinach leaves

1. Arrange onion rounds on baking sheet. Brush with oil; sprinkle with 3/4 teaspoon coarse salt and pepper. 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. Transfer to medium bowl. Toss with vinegar. DO AHEAD:
2. 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.
3. Shape beef into six 1/2-inch thick patties. Sprinkle patties on both sides with coarse salt and pepper.
4. 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. Place muffin bottoms on plates; spread with ketchup. Top with burgers, tomatoes, if desired, spinach, and muffin tops. Serve, passing remaining ketchup separately.

red onions, olive oil, coarse kosher salt, ground black pepper, balsamic vinegar, ketchup, chipotles, balsamic vinegar, ground beef, kosher salt, cheddar cheese, muffins, tomato, fresh spinach leaves

Taken from www.epicurious.com/recipes/food/views/cheddar-burgers-with-balsamic-onions-and-chipotle-ketchup-242711 (may not work)