Position a rack in the center of the oven and heat the oven to 375°F.
Scrub the potatoes and pat them dry. Put them on a baking sheet and lightly sprinkle with salt. Bake until tender when poked with a skewer, 40 to 50 minutes. Remove from the oven and set aside until cool enough to handle.
While the potatoes are cooling, heat 2 Tbs. of the olive oil in a 12-inch skillet over medium heat. Add the onion and a pinch of salt and cook, stirring often, until softened, about 5 minutes. Add the garlic and cook for 1 minute, then add the watercress. Season to taste with salt and pepper and cook, stirring, just until the watercress wilts, 1 to 2 minutes.
Put the potatoes in a large bowl and break them into small chunks with a spoon. Add the watercress mixture and the mascarpone, mix to combine, and season to taste with salt and pepper. Divide and shape the potato mixture into 16 equal patties, about 2 inches in diameter and ½ inch thick. (The cakes can be made to this point up to 4 hours ahead and refrigerated.)
Heat the oven to 200°F. Lightly oil a rimmed baking sheet. Heat the remaining 3 Tbs. olive oil in a 12-inch nonstick skillet over medium heat. Working in 2 batches, cook the potato cakes until golden on both sides, 3 to 4 minutes per side. Transfer each batch to the baking sheet, season lightly with salt, and keep warm in the oven for up to 30 minutes before serving.
nutrition information (per serving):
Photo: Christopher Hirsheimer; food styling by Melissa Hamilton