• 1 Preheat a grill to medium. ln a large nonstick skillet, heat 2 teaspoons olive oil over medium heat.
  • 2 Add the mushrooms; season with pepper and cook, turning occasionally, until browned and tender, about 5 minutes.
  • 3 Add 2 tablespoons water and cook, turning the mushrooms once, until the water is evaporated.
  • 4 Transfer to a cutting board and let cool.
  • 5 Rince the skillet.
  • 6 Finely chop the mushrooms and transfer to a bowl.
  • 7 Add the remaining 2 teaspoons olive oil and the onion to the skillet.
  • 8 Season with pepper and cook over medium heat, stirring, until the onion is tender and golden, about 3 minutes;
  • 9 Let cool. Add half of the onion and the ground beef to the mushrooms.
  • 10 Season with salt and mix well. Form the mixture into a ball, transfer to a board and cut into 8 portions.
  • 11 Form each into 3- to 4—inch patties.
  • 12 Transfer the remaining onion to a small bowl, then stir in the mayonnaise and tomato paste.
  • 13 Spread the sauce on both faces of the rolls and arrange open—faced on each of the plates. Divide half of the lettuce among the roll bottoms.
  • 14 Add the patties to the grill and cook for about 2 minutes.
  • 15 Flip, divide the cheese among the patties, cover with foil and cook until melted, about l minute.
  • 16 Stack 2 patties on each roll bottom, top with the pickle slices and remaining lettuce and set the roll tops in place.