Adjust oven rack to lower-middle position and heat oven to 325 degrees. Heat oil in a large pot over medium-high heat; add onions and peppers and sauté until tender, about 5 minutes. Add beans and remaining ingredients bring to a simmer. Pour beans into a 13-by 9-inch (or similar size) ovenproof pan. Bake until beans are bubbly and sauce is the consistency of pancake syrup, about 2 hours. Let stand to thicken slightly and serve.