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1
Preheat a grill to medium.
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2
ln a large nonstick skillet, heat 2 teaspoons olive oil over medium heat.
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3
Add the mushrooms; season with pepper and cook, turning occasionally, until browned and tender, about 5 minutes.
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4
Add 2 tablespoons water and cook, turning the mushrooms once, until the water is evaporated.
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5
Transfer to a cutting board and let cool.
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6
Rince the skillet.
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7
Finely chop the mushrooms and transfer to a bowl.
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8
Add the remaining 2 teaspoons olive oil and the onion to the skillet.
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9
Season with pepper and cook over medium heat, stirring, until the onion is tender and golden, about 3 minutes;
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10
Let cool.
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11
Add half of the onion and the ground beef to the mushrooms.
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12
Season with salt and mix well.
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13
Form the mixture into a ball, transfer to a board and cut into 8 portions.
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14
Form each into 3- to 4inch patties.
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15
Transfer the remaining onion to a small bowl, then stir in the mayonnaise and tomato paste.
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16
Spread the sauce on both faces of the rolls and arrange openfaced on each of the plates.
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17
Divide half of the lettuce among the roll bottoms.
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18
Add the patties to the grill and cook for about 2 minutes.
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19
Flip, divide the cheese among the patties, cover with foil and cook until melted, about l minute.
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20
Stack 2 patties on each roll bottom, top with the pickle slices and remaining lettuce and set the roll tops in place.
