Basic burgers are boring and bland. I didn't used to think that, but sure do now. This burger may be the best we've ever had. Three steps here:
Saute onion rounds in olive oil; sprinkle with 3/4 teaspoon coarse salt and pepper. toss in 1/2 cup vinegar and cool until onions are tender, about 20 minutes. Can be made 3 days ahead. Cover; chill.
For chipotle ketchup:
Mix ketchup and canned chipotle chiles in small bowl. Season with salt. Can be made 3 days ahead. Cover and chill.