Bring a large pan of salted water to the boil and cook the rigatoni until al dente.
Season the steaks generously with salt and pepper.
Get a frying pan hot, add a little oil and sear the steaks for 2–3 minutes each side for medium rare. Rest for at least 5 minutes, then slice.
In the same pan, reduce the heat slightly, add the butter and diced onion. Cook until softened.
Add the garlic and cook for 30–60 seconds.
Pour in the chicken stock, let it reduce slightly, then add the cream.
Stir in the parmesan and let it gently bubble until thickened.
Add the drained rigatoni with a splash of pasta water, toss until glossy and coated.
Stir through the parsley and season to taste.
Plate up and lay the sliced sirloin over the top. Finish with extra parmesan.