For the tomato sauce: In a food processor, puree the tomatoes to a smooth, creamy consistency. Heat the oil in a large pan over medium heat. Add the onions and saute until they are soft; add the garlic as soon as the onions start browning a bit. If you'd like to make the sauce a bit spicy add a pinch of red pepper.
Add the tomatoes and season with salt and pepper. Simmer over medium-low heat until the sauce thickens a bit; it needs to be fluid but not overly thin, about 30 minutes. A few minutes before taking the sauce off the heat, add half the basil and season with salt and pepper.
For the Peposo dell' Impruneta: Add the garlic, tomato sauce and beef to a large Dutch oven. Stir in the wine, salt and pepper and bring to a boil, then reduce the heat to a simmer. Cover and cook until the sauce has thickened and the meat is cooked throughout, 3 1/2 to 4 hours. Sprinkle with the remaining basil before serving.