Start by prepping your tomatoes. Peel the skin off of your tomatoes. If you have a food mill, you can avoid this step. Trick for easier peeling: dunk the tomatoes in boiling hot water for a minute and immediately cool them down in cold water, then drain. This should make peeling the skin easier.
Once your tomatoes are skinned, cut in large chunks and remove seeds and water.
Peel and slice onions and garlic. Heat a pan or skillet on medium. When hot, add some olive oil, then add sliced onions. Fry until translucent, stirring regularly. Add sliced garlic cloves and quickly stir in, just enough to start the garlic scent. Then, add in chopped tomatoes, season with salt and pepper, stir in well and lower the temperature. Cook on low for about 15-20 minutes, with lid on, stirring occasionally.
Once your sauce reduces and the tomatoes are soft enough to break them down with a wooden spoon, add some chopped basil, then with a stick blender blend your sauce to your desired texture. If your sauce is still watery, return it to the pan and cook it a bit longer, until it reduces more.
Serve on pasta immediately, or store in an air-tight container in the fridge for up to a week and use in other recipes.