Pat duck dry with paper towels; season all over with salt and pepper. (TIP: If duck skin is very thick, cut shallow slits into skin in a crosshatch pattern.) Add to a large pan, skin sides down, and heat over medium heat. Cook until fat has rendered and skin is browned and crisp, 15-20 minutes, pouring off fat as it renders. Flip and cook to desired doneness, about 5 minutes more.

Meanwhile, wash and dry all produce. Peel, then dice potatoes into 1/2-inch pieces. Place potatoes in a medium pot with enough salted water to cover by 2 inches. Bring to a boil and cook until very tender, about 15 minutes. Drain.

Strip 1 tsp thyme leaves (2 tsp for 4 servings) from stems; discard stems. Mince garlic. Halve tomatoes and lemon. Halve, peel, and mince shallot until you have 1/4 cup (1/2 cup for 4; use any remaining shallot as you like).

Melt 2 TBSP butter (4 TBSP for 4 servings) in pot used to cook potatoes over medium heat. Add garlic and cook, stirring, until fragrant, about 30 seconds. Add potatoes, 1/3 cup of milk (2/3 cup for 4), salt, and pepper. Mash until smooth and creamy. (TIP: Add more milk, as necessary, until potatoes reach a creamy consistency.) Cover to keep warm.

Once duck has finished cooking, transfer to a cutting board to rest. Wipe out pan, then return to medium heat with a drizzle of oil. Add minced shallot and thyme. Cook until shallot is softened, about 3 minutes. Add demi-glace, jam, and 1/4 cup water (1/3 cup for 4 servings), stirring to break up jam. Let simmer until thick and glossy, about 2 minutes (2-3 minutes for 4). Stir in a squeeze of lemon juice. Season with pepper and more lemon juice (to taste).

In a medium bowl, combine watercress, tomatoes, a squeeze of lemon juice, and a large drizzle of olive oil; toss to thoroughly coat. Season with salt and pepper. Thinly slice duck crosswise. Divide potatoes and duck between plates, then drizzle duck with sauce. Serve with salad on the side.