Salad: 2 tablespoons Dijon mustard 1 tablespoon best-quality red wine vinegar ½ small garlic clove, finely grated Kosher salt, freshly ground pepper ¼ cup olive oil 2 small bunches watercress, tough stems trimmed (about 10 cups)
STEPS Chicken: Pat chicken dry with paper towels and season generously with salt, inside and out. (We use 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt per lb.) Tie legs together with kitchen twine. Let sit 1 hour to allow salt to penetrate, or chill, uncovered, up to 1 day ahead.
Place a rack in upper third of oven and set a 12" cast-iron skillet or 3-qt. enameled cast-iron baking dish on rack. Preheat oven to 425°.
Meanwhile, toss potatoes, butter, thyme, and 1 Tbsp. oil in a large bowl to coat; season with salt and pepper.
Once oven reaches temperature, pat chicken dry with paper towels and lightly coat with half of remaining oil; sprinkle with dry rub, if using. Drizzle remaining oil into hot skillet (this helps keep the chicken from sticking and tearing the skin). Place chicken in the center of skillet and arrange potatoes around. Roast until potatoes are golden brown and crisp and an instant-read thermometer inserted into the thickest part of breasts registers 155°, 50–60 minutes (temperature will climb to 165° as chicken rests). Let chicken rest in skillet at least 20 minutes and up to 45 minutes.
Transfer chicken to a cutting board and carve. Serve with potatoes.
Salad: Whisk mustard, vinegar, and garlic in a medium bowl; season with salt and pepper, then gradually stream in oil, whisking constantly. Whisk until dressing is emulsified and thick.
Add watercress to dressing, season with more salt and pepper, and gently toss until watercress is evenly coated (you want to be careful not to bruise or wilt greens when tossing, so use a light hand). Pile salad lightly onto plates.
