Cheddar Burgers with Balsamic Onions and Chipotle Ketchup

Ingredients

• 1 pound red onions
• 1 ½ teaspoons chipotle chiles from canned chipotles in adobo
• 2 tablespoons balsamic vinegar
• ½ teaspoon coarsely ground black pepper
• ¾ teaspoon coarse kosher salt
• 1 cup ketchup
• 2 teaspoons balsamic vinegar
• + 8 more ingredients
• • 2 ¼ pounds ground beef (15% to 20% fat)
  • Coarse kosher salt
  • 6 thick slices sharp cheddar cheese
  • 6 large English muffins or hamburger
  • 6 tomato slices
  • 2 tablespoons adobo sauce from can
  • 2 cups fresh spinach leaves
  • Olive oil

Prepare barbecue (medium-high heat): 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 coo...

View full recipe at Epicurious