Preparation

• In a medium Dutch oven or other heavy-duty pot, heat the oil over medium heat until shimmering. Add the onion and cook, stirring occasionally, until softened and translucent, about 3 minutes. Add the garlic and cook until fragrant, about 1 minute. Add the beans and cook, stirring occasionally, until the onion and garlic just begin to brown and the beans turn bright green, about 3 minutes. Add the wine, raise the heat to high, and cook, stirring occasionally, until the pot is almost dry and the wine is mostly evaporated, 7 to 8 minutes. Add the tomatoes, peppers, and 1 tsp. salt. Bring to a boil, then reduce to a simmer, breaking up larger tomato pieces with a wooden spoon.
• Reduce the heat to very low, cover with the lid slightly ajar, and cook, stirring occasionally, until the beans are tender, 60 to 75 minutes. Remove from the heat, and stir in the butter one piece at a time until it melts. Season to taste with salt, transfer to a large bowl, and serve.