Heat oven to 450°. Place potatoes in a large pot and cover with 1†cold water. Season with salt and bring to a boil. Reduce heat and simmer until tender, 15–18 minutes. Drain and return to pot to dry. Working with one potato at a time, place each under a glass or flat-bottom measuring cup and press to flatten. Divide oil between two rimmed baking sheets. Transfer smashed potatoes to baking sheets and turn to coat. Season with salt, pepper, and ginger. Roast, flipping once, until golden, 20–25 minutes. Meanwhile, whisk together buttermilk, mayonnaise, brown sugar, thyme, and oregano; season with salt and pepper. Fold warm potatoes into dressing along with watercress and scallions; season with salt and pepper.