Recipe

1. For onions: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 cooler part of grill. Close cover; cook until onions are tender, about 10 minutes. Transfer to medium bowl. Toss with vinegar. DO AHEAD: Can be made 3 days ahead. Cover; chill.

2. For chipotle ketchup: Mix ketchup, chiles, 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.

3. 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. Place muffin bottoms on plates; spread with ketchup. Top with burgers, tomatoes, if desired, spinach, and muffin tops. Serve, passing remaining ketchup separately.

4. *Dried, smoked jalapeños in a spicy tomato sauce called adobo; available at some supermarkets and at specialty foods stores and Latin markets.