Boil the pasta: Bring a large pot of salted water to a boil. Add the penne and cook until al dente according to package instructions. Reserve ½ cup of pasta water, then drain.
Sauté garlic and chili: Heat olive oil in a deep pan over medium heat. Add the sliced garlic and chili flakes. Cook for 1–2 minutes, just until fragrant and lightly golden — don’t burn the garlic.
Add the tomatoes: Pour in the crushed tomatoes, season with salt and pepper, and stir. Let simmer uncovered for 8–10 minutes until the sauce slightly thickens.
Combine pasta and sauce: Add the drained penne to the sauce. Toss well to coat. If the sauce feels too thick, add a splash of the reserved pasta water.
Finish with parsley: Remove from heat and stir in the freshly chopped parsley. Adjust seasoning if needed.
Serve hot: Plate the pasta, sprinkle with grated Parmesan, and enjoy immediately — a true Roman-style spicy classic.