Melt the butter and olive oil in a large pot over medium heat. Add the onions and cook, stirring, just until they turn translucent, 5 to 7 minutes. Add the fennel, celery, leeks, carrots, and tomatoes and cook until the vegetables are tender, about 15 minutes. Add the saffron, sachet, heavy cream, mussel liquid and chicken stock. Taste and season with salt and pepper. Bring the mixture to a boil and cook for 20 minutes until all the vegetables are tender. Skim the soup to remove any foam as it cooks.