Vary the vegetables according to what is available. Just chop it all into small, similar-sized pieces and "build" the soup, cooking hte harder, denser vegetables for slightly longer, and you'll end up with a vibrant, fresh-tasting bowlful.
Chop the fennel, celery, green onions, and zucchini into small dice, keeping them separate. Tear the leaves from the chard stalks and shred them. Cut the stalks into small pieces.
Heat a saucepan over medium heat and add the butter and oil. Add the fennel, celery, green onions, and chard stalks and cook gently for about 10 minutes, until soft but not colored. Add the stock and bring to a simmer. Season with salt and pepper.
Make sure the broth is simmering well and add the zucchini. Once returned to a simmer, cook for 1 minute, then add the peas and fava beans. Simmer for another 2 minutes. Check that the peas and beans are just tender, then add the lettuce and the shredded chard leaves. Simmer for another minute.
Add the chopped herbs along with any feathery fennel fronds and, if you have them, the pea shoots, then immediately remove from the heat. Check the seasoning, then ladle into warmed bowls and serve.