Cheddar Burgers With Balsamic Onions And Chipotle Ketchup

INGREDIENTS

2 1/4

lbs Ground beef

1 lb

Red onions

2 cups

Spinach, fresh leaves

Tomato

2 2/3 tbsp

Balsamic vinegar

1 1/2 tsp

Chipotle chiles from canned chipotles in adobo* plus 2 tabelspoons adobo sauce from can

1 cup

Ketchup

1/2 tsp

Black pepper, coarsely ground

3/4 tsp

Kosher salt, coarse

Kosher salt, Coarse

Olive oil

English muffins or hamburger buns, large

thick slices Cheddar cheese, sharp