Debone the sea bass and use the bones for a fresh fish stock. Finely slice the fennel, onions, leeks and garlic and simmer on very low heat until vegetables is softened. Add the saffron and toss, add the white wine and stock bring to the boil and simmer by half. Once broth is reduced add the mussels, lemon zest and quartered cherry tomatoes stir for a few minutes and remove from heat, finish with chopped dill. Roll the sea bass in cling film and simmer with the cling film in fish stock. Cut the potatoes and boil while sauté the spinach leaves to serve underneath with the mussels. Use the lettuce to garnish at the top.