Step 1

Melt butter in a large heavy skillet; add pancetta. Cook until pancetta is rendered and beginning to brown, 4–5 minutes. Add onion; cook until onion is soft and pancetta is crisp, 3–4 minutes. Meanwhile, cook pasta in a large pot of boiling, lightly salted water, stirring occasionally, until al dente. Drain pasta, reserving 1 cup cooking liquid.

Step 2

Add reserved cooking liquid and miso to skillet; bring to a simmer. Add pasta; simmer, tossing constantly, until sauce is glossy and coats pasta, 1–2 minutes. Add parsley and pepper. Toss to coat. Garnish with mozzarella and additional chopped flat-leaf parsley.