Ingredients

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

Chipotle ketchup:

• 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

Burgers:

• 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

Description

The All-American Cheeseburger Gets A Makeover, Courtesy Of Grilled Onions, A Spicy-smoky Ketchup, And An English Muffin 'bun.'

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