CHEDDAR BURGERS WITH BALSAMIC ONIONS AND CHIPOTLE KETCHUP

Onions:
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 chilies 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 focaccia buns or hamburger buns, split, cut sides grilled
6 tomato slices (optional)
2 cups fresh spinach leaves

For onions:
Place ingredients for onions in a zip lock and shake to mix. Transfer onion rounds to baking sheet; Bake at 400 for about 20 - 30 minutes until roasted. Remove from baking sheet and let cool. DO AHEAD: Can be made 3 days ahead. Cover; chill.

For chipotle ketchup:
Mix ketchup, chilies, 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.

For burgers:
Shape beef into six 1/2-inch thick patties. Sprinkle patties on both sides with coarse salt and pepper. Prepare barbecue (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. Toast buns on grill. Place bun bottoms on plates; spread with ketchup. Top with burgers, tomatoes, if desired, spinach, and muffin tops. Serve, passing remaining ketchup separately.