Burgers with Blue Cheese

INGREDIENTS

• 1 red onion, thinly sliced
• 1/4 cup red wine vinegar
• 1/4 cup red wine
• 1/2 cup sugar
• 1 Tbs. fresh thyme leaves
• 1/4 cup light agave syrup
• 1 1/2 lb. ground beef chuck or sirloin (75% to 80% lean)
• Salt and freshly ground pepper, to taste
• 3 to 4 Tbs. very cold water
• 4 Tbs. unsalted butter, melted
• 4 sandwich rolls, such as seeded Kaiser, split
• 4 to 5 oz. blue cheese, sliced or crumbled
• 1 to 2 cups shredded crisp lettuce leaves, such as iceberg or romaine
• Chipotle ketchup for serving (see related recipe at left; optional)
• Beer mustard for serving (see related recipe at left; optional)