Preheat oven to 450 degrees, with racks on upper and lower thirds. On a rimmed baking sheet, toss sweet potatoes with 1 tablespoon oil; season with salt and pepper. Roast on upper rack, until tender, 30 to 35 minutes, tossing halfway through.
On another rimmed baking sheet, toss walnuts with cayenne and 1/2 teaspoon oil. Bake on lower rack,
until golden, 6 minutes.
In a medium bowl, whisk together lemon juice, honey, and remaining 2 tablespoons oil; season with salt and pepper. Add watercress, and toss to combine. Serve topped with sweet potatoes, walnuts, and feta.