1. 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.
2. Cook until grill marks appear, about 4 minutes per side. Reduce heat or move onions to cooler part of grill. Close cover; cook until the onions are tender, about 10 minutes.
3. Transfer to medium bowl. Toss with vinegar. DO AHEAD Can be made 3 days ahead. Cover; chill.
1. Mix ketchup, chiles, adobo sauce, and 2 teaspoons vinegar in small bowl. Season with salt and more vinegar, if desired. DO AHEAD Can be made 3 days ahead. Cover and chill.
BURGERS/
Shape beef into six 1/2-inch-thick patties. Sprinkle patties on both sides with coarse salt and pepper.
1. 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.
2. Top with onions and cheese. Close cover, cook until cheese melts.
3. Place muffin bottoms on plate; spread with ketchup. Top with burgers and tomatoes, if desired, spinach and muffin tops. Serve, passing remaining ketchup separately.
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