Place 12 ounces dried spaghetti, 1 (32-ounce) box chicken broth, ½ cup water, and 1 Parmesan cheese rind, if using, in a large pot. Bring to a boil over high heat and cook uncovered, stirring occasionally, until the pasta has absorbed all but about ⅔ cup liquid, about 9 minutes.
Add ¾ cup grated Pecorino-Romano cheese and 2 teaspoons fresh, coarsely ground black pepper, and continue cooking, stirring constantly, until the pasta is coated with a silky sauce, about 2 minutes.
Discard the Parmesan rind and serve immediately, topping with more Pecorino if desired.
