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1
In a medium bowl, gently mix together the beef, soy sauce, 3/4 tsp of the pepper, 1/2 tsp of the salt.
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2
Using generous 1/3 cup per patty, shape mixture into 8 (4 1/2-inch) patties 3/8 inch thick.
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3
Refrigerate while preparing the stuffing.
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4
Heat a large nonstick skillet over medium high heat until hot.
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5
Add the butter and oil.
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6
Heat until hot and butter is melted.
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7
Add the mushrooms and cook for 2 min, stirring occasionally.
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8
Add the garlic and cook for 8-10 min or until the mushrooms are browned and the liquid has evaporated.
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9
Stir in the green onions, Worcestershire sauce, remaining 1/2 tsp pepper and remaining 1/4 tsp salt.
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10
Let cool.
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11
Mound about 1/2 cup of the stuffing in the center of 4 of the patties.
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12
Cover with the remaining 4 patties, sealing the edges firmly by pressing together with your fingers.
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13
Gently press the center of the burgers to eliminate mounding.
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14
Heat the grill or grill pan to medium heat.
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15
Grill for 5-6 minute.
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16
Flip the burgers and grill for 5-6 min more, or until no longer pink in the center.
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17
Top each burger with 1 slice of cheese, and grill for another min or so - until the cheese has melted.
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18
Meanwhile, in a small bowl, whisk together all of the mayonnaise ingredients.
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19
Place the buns on the grill, split side down, and grill for 30-60 sec or until lightly toasted.
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20
Spread each bun with 2 Tbs of the mayonnaise.
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21
Top with a burger and a tomato slice.
