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1
Prepare a medium-high-heat fire, with the flames occasionally licking the grill grate.
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2
Let it burn steadily for 30 minutes.
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3
Place a large cast-iron skillet upon the grill grate and let it heat until very hot.
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4
Score the fatty skin of the duck in a 3/4-inch diamond pattern, taking care not to cut through to the flesh.
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5
Season the duck all over with salt and pepper.
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6
Transfer the duck, skin side down, to the hot skillet and cook until it is honey-brown and crisp, 5 to 8 minutes.
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7
Pour off all but 2 tablespoons of fat into a heatproof container (reserve it for Duck Eggs with Rosemary Salted Potatoes).
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8
Turn the duck and adjust one's grill grate or tamp one's flames so that the fire provides medium heat.
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9
Cook to the desired doneness, 4 to 7 minutes (depending upon the size of the breast) for medium rare.
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10
Transfer the duck to a work surface, tent it with foil, and let rest for 10 minutes.
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11
Again pour off and reserve all but 2 tablespoons of the fat drippings.
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12
Return the skillet to the heat.
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13
Stir in the shallots and cook 30 seconds.
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14
Add the port, broth, plums, and vinegar.
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15
Simmer until the mixture thickens slightly, 2 to 3 minutes.
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16
Whisk in the butter and fresh thyme; season to taste with salt and pepper.
