Shell beans take less than half the time to cook than their dried counterparts. Look for varieties such as mottled pink and white cranberry beans, also known as borlotti beans; large scarlet runner beans that are mottled and purple, despite the name; and pale yellow cannellini beans.
In a heavy soup pot or Dutch oven, combine the shell beans, onion, crushed garlic clove, bouquet garni and 4 cups water. Bring to a boil. Add salt to taste, reduce heat, cover and simmer 30 minutes. Remove and discard onion and garlic clove.
02Meanwhile, heat olive oil over medium heat in a medium skillet and add leeks and celery. Cook, stirring, until they begin to soften, about 3 minutes, and stir in minced garlic. Cook, stirring, until fragrant, 30 seconds to a minute, and add tomatoes. Season to taste with salt and pepper. Cook, stirring often, until tomatoes have cooked down slightly, about 10 minutes.
03Stir leek and tomato mixture into beans, along with summer squash. Bring back to a simmer and cook 15 minutes, until beans are creamy-tender but intact and the vegetables are soft and fragrant. Taste and adjust seasonings. Remove bouquet garni.
04Ladle ragout into wide bowls. Top each serving with a generous sprinkling of slivered basil leaves and a spoonful of Parmesan.