Bring a large pot of well-salted water to a boil. Meanwhile, chop the garlic, sprinkle with 1/2 tsp. salt, and mash to a paste with the flat side of a chef's knife. Set aside.
Cook the pasta according to the package directions until al dente. Reserve about 1/3 cup of the cooking water, drain the pasta in a colander, and return to the pot.
While the pasta cooks, heat the oil in a 12-inch cast-iron or nonstick skillet over medium heat until hot. Add the pancetta and cook, stirring frequently, until the fat is rendered, 4 to 6 minutes. Stir in the garlic and pepper flakes and cook, stirring constantly, until fragrant, about 30 seconds. Stir in the tomatoes and cook, stirring frequently, until softened, 3 to 5 minutes. Remove the skillet from the heat and add the watercress; stir until wilted, 1 to 2 minutes.
Add the watercress mixture and pecorino to the pasta. Stir to combine, adding the reserved water 1 Tbs. at a time as needed to moisten. Season to taste with more salt and serve.