Mix beef, pork, egg, 1/2 chopped mushrooms, capers, allspice, nutmeg and parmesan cheese in a large bowl.
Shape into burger patties.
Heat butter and olive oil in a large skillet.
Brown burgers at medium-high heat for 3-4 minutes on each side, till crispy and dark on the outside.
Remove burgers to a platter, set aside and keep warm; reserve the oil and drippings in the skillet; prepare the sauce.
On medium-high heat, saute the mushrooms and the shallot in skillet with the oil left from cooking the burgers, until mushrooms are soft and the shallot is transparent, approximately 8 minutes. Do not brown.
Turn the heat to high, add the wine, and stir until the wine has burned off.
Add the beef broth, sour cream, nutmeg and salt and pepper and simmer on low heat until the sauce is reduced by a least half. The consistency should be similar to ketchup or mustard.
Assemble the burgers on the buns, topping with sauce and carmelized onions and peppers. Share and enjoy!