Prep the Pasta — Cook the penne in a large pot of aggressively salted water according to the package directions until just al dente. Do not overcook; the pasta will finish for a minute in the sauce and you want it to retain a "bite."
Render the Fat — In a large skillet, heat the olive oil over medium and add the diced pancetta. Stir occasionally until the fat turns translucent and the edges start to crisp and turn golden brown.
Aromatic Base — Toss in the sliced garlic and chopped onions, sautéing until the onions are soft and the garlic is fragrant but not brown. Watch the heat here; if the garlic smells "toasty," move to the next step immediately to avoid bitterness.
Sear the Mushrooms — Increase the heat slightly and add the sliced mixed mushrooms to the pan. Let them sit undisturbed for a minute to get a hard sear before stirring; they should hiss and eventually release their moisture and shrink.
Build the Sauce — Stir in the tomato paste, the drained/crushed San Marzano tomatoes, and the sun-dried tomatoes. Use your spoon to break up the whole tomatoes against the side of the pan until the sauce looks thick and chunky.
Simmer and Season — Lower the heat and let the sauce bubble gently for about 10 minutes until it darkens in color. Season with kosher salt and a generous amount of freshly ground black pepper, tasting to ensure the salt from the pancetta is balanced.
The Marriage — Drain the penne and toss it directly into the skillet with the sauce. Stir vigorously so the sauce enters the tubes of the pasta and coats every ridge.
The Final Touch — Remove from heat and shower the pan with grated Parmigiano-Reggiano and minced fresh parsley. The cheese should melt into the sauce, creating a slight gloss, while the parsley provides a burst of green color.