Put the tomatoes, onion and butter in a heavy pot.
Bring the sauce to a simmer then reduce the heat to keep the sauce at a gentle simmer for about 45 minutes, (until droplets of fat are released from the tomatoes).
Stir occasionally, crush the tomatoes and onion against the side of the pot with a wooden fork.
Remove from heat, add salt if needed.
Prepare the pasta of your choice.
Put a generous serving of sauce on each bowl of pasta.
Serve with freshly grated Parmigiano Reggiano and chiffonade of basil.
A glass of wine would be lovely.
Notes
Solania Pomodoro dell'Agro Sarnese - Nocerino D.O.P. is what is on the San Marzano tomatoes in Sweden near Stockholm. I found them at ICA Kvantum in Skärholmen. Kungens Kurva...
Recipe by at http://trotsig.com/spaghetti-sauce-from-heaven/