Pat duck breast dry. Using a sharp knife, score the fat in a crosshatch pattern without cutting into the meat.

Season generously with kosher salt and garam masala. Let sit at room temperature for 20 minutes.

In a small saucepan, combine chopped plums, honey, vinegar, soy sauce, ginger, garlic, water, and a pinch of pepper.

Bring to a simmer over medium heat and cook until the plums break down and the sauce thickens (10–12 minutes).

Taste and adjust with more honey or vinegar as needed. Keep warm.

Place the duck breast skin side down in a cold cast iron skillet. Turn heat to medium and let the fat render slowly, about 8–10 minutes, until the skin is deep golden and crispy.

Flip the duck and cook the flesh side for 2–4 minutes, depending on thickness, for medium rare (internal temp: 130°F).

Rest on a board for 5–7 minutes before slicing.

Slice duck thinly against the grain. Plate with a generous spoon of plum sauce. Finish with flaky salt if desired.