Finely chop the dry onions and the mushrooms and sauté them in a bit of olive oil. Clean the artichokes, rub them with lemon and finely chop them. Add the artichokes and the vegetable broth. Add salt and pepper and boil over a medium heat for about 30 minutes. When the vegetables boil, mix in 1/4 cup of water, the cornflour, lemon juice, finely chopped dill and finely chopped onions. Add this mixture in the soup, stir and leave over a medium heat for another 5 minutes. Add salt or lemon if needed and serve.