ONIONS:

2 tablespoons butter

1 large onion, quartered lengthwise then thinly sliced

2 tablespoons spicy mustard

1 1 pound package sauerkraut, rinsed and drained

SOUR CREAM SAUCE:

3/4 cup sour cream

1/4 cup good-quality ketchup, such as Heinz Organic

1/4 cup dill relish

Kosher salt and pepper

BURGERS:

1 1/2 pounds ground mix of beef, pork, and veal

1 teaspoon celery seeds

1 teaspoon dried marjoram

1 teaspoon ground ginger

1/2 teaspoon freshly grated nutmeg

1/2 teaspoon ground cardamom

1/4 cup finely chopped flat-leaf parsley (a handful)

2 tablespoons cider vinegar or white wine vinegar

1 tablespoon EVOO (extra-virgin olive oil)

4 slices deli-sliced Emmentaler Swiss cheese, folded to fit burgers

4 seeded or plain burger rolls, split