1Cook 1 lb. spaghetti in a large pot of boiling heavily salted water, stirring occasionally, until very al dente (about 1 minute less than package instructions; pasta will finish cooking in the sauce). Drain, reserving 1 cup pasta cooking liquid. Shake pasta to remove excess water. Reserve pot.
2Heat ¼ cup extra-virgin olive oil in reserved pot over medium. Drain one 15.5–16-oz. jar peperoncini in a mesh sieve over a small bowl; set brine aside. Finely chop peperoncini (you should have about 1 cup). Add to pot along with 3 oil-packed anchovy fillets, coarsely chopped (if using), and cook, stirring often, until peppers brown slightly and are tender, 7–9 minutes.
3Reduce heat to low and add 14 garlic cloves, finely grated, and cook, stirring often, just until fragrant, about 1 minute. Add 1½ oz. Parmesan, finely grated (about ¾ cup), 4 Tbsp. unsalted butter, cut into pieces, reserved pasta, and ½ cup reserved pasta cooking liquid. Cook, stirring and adding more pasta cooking liquid a splash at a time if pasta looks dry or cheese is not combining, until cheese is melted and emulsified and pasta is coated and glossy. Add ⅓ cup reserved brine and remaining 2 Tbsp. extra-virgin olive oil and toss to coat. Taste pasta and add more brine and/or season with salt as needed. (Some brines are very spicy, so add just a splash more at a time. Depending on how salty your brine is, you might not need to add any salt.)
