, (about 400 g), patted dry
PLACE potatoes in a medium pot filled with cold, salted water. Bring to a boil and cook until just tender, 10 to 12 min. Drain.
COOK bacon in a large cast iron frying pan over medium until crispy, 7 to 8 min.
WHISK vinegar, shallot, Dijon and 1/4 tsp each salt and pepper in a large bowl. Whisk in bacon and any drippings, then stir in potatoes.
WIPE frying pan clean, then heat oil over medium. Sprinkle both sides of mackerel with 1/2 tsp each salt and pepper. When pan is very hot, add fish, skin-side down. Press down with a spatula so skin contacts pan. Cook until skin is crispy, about 5 min. Flip and cook until flesh flakes easily, about 1 more min.
STIR watercress and parsley into potatoes. Serve alongside mackerel.