Into a large saute pan, add your oil and butter and heat over medium heat. Add your onions, garlic, tomatoes and basil. Stir together and season with salt. Reduce to LOW and cook for approximately 15-20 minutes.

WHILE sauce is cooking, place a large pot of water on to boil. Season water generously with salt. When water is boiling, add your pasta and cook until al dente. Reserve 1/2 cup of the cooking liquid and then drain pasta and set aside briefly. When sauce has finished cooking, transfer it into your food processor, food mill, blender, or use your immersion blender (whatever you have available) and pulse until it has a smooth and silky consistency. Return to the pan and mix in the mascarpone. Check for seasoning and salt if necessary.

Once blended completely, add the pasta into the sauce and toss well. Add the reserved cooking liquid if sauce is too thick. Garnish with additional basil if desired and Parmigiano Reggiano!