Warm up medium sized pot over medium heat. Add in oil and butter. When warmed up, slowly toss in onions and garlic. Sauté for 2-3 minutes until onions are becoming translucent.
Add in San Marzano tomatoes and sauce. Gently crust tomatoes with your hand until they are in small pieces! You'll want to do this right away so the tomatoes don't get too hot! Season with salt and pepper, and a little bit of salt.
Cook down for 20 minutes. Remove from heat let cool for 5 minutes. Using an immersion blender, blend until sauce is smooth, about 1-2 minutes. At this point add in cream and stir to blend. Use immediately or let cool and save for later use.