Heat 1/4 cup olive oil in a 12 inch skillet over medium low heat.
Add one minced or chopped onion, stirring until soft, about 12 minutes.
Add minced garlic cloves and cook, stirring for 2-4 minutes.
Add a pinch of crushed red pepper flakes. Cook for about 1 minute. Increase heat to medium and add can of tomatoes that have been pureed in a food processor or blender. Lightly season with kosher salt.
Cook, stirring occasionally, until sauce thickens slightly and the flavors meld, about 20 minutes.
Remove pan from heat, stir in fresh basil and set aside.
Bring 3 quarts of water to a boil in a 5 quart pot. Season with salt. Add pasta and cook, stirring occasionally until about 2 minutes before tender. Drain pasta, reserving at least 1/2 cup of the pasta cooking water.
Discard basil and heat skillet over high heat.
Stir in reserved pasta water to loosen sauce, bring to a boil. Add pasta and cook stirring until sauce coats pasta and pasta is al dente about 2 minutes.
Remove pan from heat and add the cubed butter and grated cheese. Toss with metal tongs until the cheese melts. Transfer to warm bowl or plates; serve with more grated cheese if desired.
Notes
-This can be made in a slow cooker. Place all of the ingredients in the slow cooker for the day on low. I'm guessing 6 hours will be enough time...If you pre cook the onion and then dump it all in the slow cooker, about 3 hours on low. If using the slow cooker, I would make sure to double the recipe and freeze the leftover sauce.-Long (12" or longer) metal tongs. They cost just a few dollars. You need them for this dish.- I usually like my onions chunky, but cut them a little smaller for this recipe.-One time I was out of whole peeled tomatoes, and I subbed chopped tomatoes. No problem.-If fresh basil is unavailable in your area, go ahead and use dried basil. Of course, you won't be retrieving it from the sauce before serving, as you would with the fresh sprigs. I would try about 1 tablespoon of dried basil leaves. Fresh is best, if at all possible.-I'm not a huge garlic fan so I cut back and used only 2 cloves.