Bring a large pot of water to a boil. Add 2 tablespoons of salt and return to a rolling boil.
While the water comes to a boil, prepare the sauce: Place the bacon and olive oil in a 12-inch skillet over medium-low heat. Cook until the bacon is crisp, stirring occasionally to make sure it does not burn, about 8 minutes. Remove the bacon and set aside. Pour off all but 2 tablespoons fat from the pan.
Add the pasta to the boiling water and cook until al dente, according to package directions.
Add the tomatoes to the skillet and cook over medium heat, coating them in the bacon fat. Season with salt and pepper. As the moisture from the tomatoes releases and deglazes the pan, scrape up any browned bits on the bottom of the pan. Sauté the tomatoes until they are tender and almost melt, about 5 minutes more. Add half of the bacon to the skillet and toss together to combine.
Increase the heat to medium-high. Add the pasta directly to the skillet and toss to coat. Add the watercress and ½ cup pasta water and toss until the watercress wilts, adding ¼ cup more of pasta water (up to 1 cup) as needed to loosen up the sauce.
Plate in bowls, season with salt and pepper, and top with the remaining bacon. Pass the grated Pecorino Romano at the table.