In a large bowl, cover beans with cold water; let soak overnight. (Alternatively, in a large pot, cover beans with water and bring to a boil. Boil for 2 minutes. Remove from the heat and let stand for 1 hour.)
Drain and rinse the beans; place in a large pot and cover with fresh water. Bring to a simmer and cook over low heat until almost tender, about 45 minutes.
Meanwhile, preheat oven to 300 degrees F. Transfer beans and cooking liquid to a 2-qt. casserole or bean pot. Add onions, stewed tomatoes, molasses and sun-dried tomatoes. Cover and bake for 2 to 3 hours, or until the beans are very tender and the sauce has thickened. (Add more water as needed to keep the beans barely covered.) Season with salt and pepper and serve.