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1
Bring a large pot of salted water to rolling boil; add tonnarelli and cook until 1 minute before al dente (the pasta will finish cooking off-heat).
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2
While pasta cooks, render guanciale lardons in a large shallow pan over medium heat until fat is fully released and solids are crisp (4–5 minutes); turn off heat.
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3
Combine Parmigiano Reggiano and Pecorino Romano in a small bowl; crack a generous amount of black pepper into the same bowl and mix.
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4
Reserve 200ml pasta cooking water; drain tonnarelli and add immediately to the pan with guanciale and fat (off heat).
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5
Add half the cheese mixture and toss constantly, adding pasta water one tablespoon at a time to form a silken emulsion; the residual heat will cook the pasta to al dente and create a glossy sauce.
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6
Mount the remaining cheese and pepper, tossing off heat until fully incorporated; serve immediately in warmed bowls with additional pepper on top.
