Step 1

In a small bowl combine sour cream, mayonnaise, horseradish, salt, and pepper. Mix well. Place into the refrigerator for at least 30 minutes.

Step 2

Melt the butter and oil in a large sauté pan over low heat. Add in the sliced onions, and the dried thyme and cook slowly for about 20 minutes until the onions are caramelized. Season with salt and pepper.

Step 3

Season the burger patties on both sides with salt and pepper. Heat a large skillet to medium high heat. Add the burger patties and cook, turning frequently until fully cooked (the internal temperature has reached 160 degrees F), about 5 minutes per side or more, depending on the thickness of the patties. During the last few minutes of cooking, top with a slice of provolone cheese and cook until the cheese has melted.

Step 4

Serve on buns with horseradish cream and caramelized onions.