Cook pasta in salted, boiling water until "al dente." Drain.
While pasta is cooking, cook pancetta in olive oil in a large, wide pan until crispy. Remove with a slotted spoon and set aside. If the pancetta has rendered a lot of oil, drain it off so that you have about 1 tablespoon of fat in the pan.
Add your jarred La San Marzano pasta sauce of choice and bring to a simmer. Add mussels and toss to coat them in the sauce. Cover and simmer for 5-7 minutes, until the mussels' shells have opened. Start checking the mussels around 3-4 minutes, and use tongs to remove the open mussels to a bowl while the others continue to cook. Discard any mussels that do not open.
When all the mussels are cooked and removed from the pan, increase heat and simmer the sauce, stirring, until slightly thickened, about 5 minutes. Stir pancetta back into the sauce and toss with the pasta.
Nestle mussels into the pasta. Drizzle with a little olive oil and serve, sprinkled with fresh parsley.