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1
Melt 2 tablespoons of the butter in a large frying pan over medium heat.
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2
Add the onion slices and a pinch of salt and cook, stirring occasionally, until the onions are evenly golden brown and tender, about 20 minutes.
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3
In a medium bowl, combine the turkey, chopped onion, Worcestershire sauce, and mustard.
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4
Season with salt and pepper.
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5
Use your hands to gently combine.
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6
Divide the meat into 4 equal portions.
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7
Shape each portion into an even 1/2-inch-thick patty.
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8
Using your thumb, make a shallow indentation in the center of each patty (this helps them cook evenly).
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9
Transfer the patties to a plate and set aside.
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10
Set a grill pan over medium-high heat.
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11
When its hot, spray with cooking spray.
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12
Use a metal spatula to transfer the turkey patties to the pan and cook, undisturbed (dont be tempted to press down with the spatula!
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13
), until grill marks appear on the bottom, about 5 to 6 minutes.
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14
Flip the patties and continue cooking until theyre cooked through and an instant-read thermometer registers 165 degrees F, about 6 minutes more.
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15
Spread the remaining 2 tablespoons of butter on one side of each bread slice.
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16
Arrange the slices buttered-side down on a work surface.
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17
To each, add a slice of cheese, a turkey patty, a quarter of the caramelized onions, a second slice of cheese, and a second slice of bread, buttered-side up.
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18
Place a large two-burner griddle or large frying pan over medium heat until a drop of water sizzles and immediately disappears on the surface, about 3 minutes.
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19
Place 2 of the sandwiches on the grill or in the pan and cook until the bottom slices of cheese start to melt and the bread is golden brown, about 3 to 4 minutes.
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20
Flip and cook until the other side is golden brown and all of the cheese has melted, about 3 to 4 minutes more.
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21
Remove to a serving plate and repeat with the remaining 2 patty melts.
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22
Serve immediately.
