Add the bison,
tomato paste, cumin, salt, black pepper and cayenne pepper to the onion mixture, and stir to combine. Form into four 1-inch-thick burgers. Place the
skillet back over high heat, add the burgers and cook until an instant read
thermometer registers 160 degrees F, 5 to 6 minutes per side. Remove from the pan and rest the burgers for 3 to 5 minutes.