Preheat the oven to 375°F. In a medium bowl, combine the potatoes, 3 Tbsp. of the olive oil, the lemon zest, and juice. Spread on a baking sheet and sprinkle generously with salt and pepper. Bake, stirring occasionally, until golden brown and tender, 30 to 35 minutes.

In a large pot of salted boiling water, blanch the fava beans for 1 minute. Drain in a colander and rinse under cold water to stop the cooking. Peel the skins from the beans and discard the skins. (If using soybeans, cook them in a large pot of salted boiling water until tender, about 5 minutes. Drain.)

In a medium bowl, whisk together the shallot and the remaining 2 Tbsp. olive oil. Season with salt and pepper to taste. Add the watercress and toss to coat.

Divide the watercress among 6 small plates. Top each serving with 4 potato quarters and equal amounts of fava beans. Using a vegetable peeler, shave a few slices of pecorino over each salad. Serve warm.