Cheddar Burgers with Balsamic Onions and Chipotle Ketchup

Source: Bon App├ętit - July 2008, recipe by Tony Rosenfeld
adapted by There's Always Thyme to Cook

Onions:

2 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

Chipotle ketchup:
1 1/4 cup ketchup
2 chipotle chiles, chopped, from canned chipotles in adobo plus 3 tablespoons adobo sauce from can
1 tablespoon balsamic vinegar

Burgers:
2 1/4 pounds ground beef, made into six 1/2-inch thick patties
Coarse kosher salt (for sprinkling on the burgers)
6 thick slices sharp cheddar cheese
6 large English muffins or hamburger buns, split, cut sides grilled
6 tomato slices
2 cups fresh spinach leaves or romaine leaves
slices of avocado

For onions:
Prepare grill (medium-high heat): Arrange the onion rounds on a 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. Onions can be made 3 days ahead. Cover; chill.

For chipotle ketchup:
Mix ketchup, chiles, adobo sauce, and 2 teaspoons vinegar in a small bowl. Or whiz it up in a food processor. Season with salt and more vinegar, to taste. Can be made 3 days ahead. Cover and chill..

Prepare grill (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. Top with onions and cheese. Close cover; cook until cheese melts. Place muffin bottoms on plates; spread with ketchup. Top with burgers, tomatoes, spinach, avocado and muffin tops. Serve, passing the remaining ketchup.

*You can also mix a little of the chipotle ketchup into the burger meat mix!