Heat the olive oil in a large saucepan and toast the saffron in it. Add the carrots, peppers and garlic cloves and sauté briefly. Top up with vegetable stock and bring to the boil. Add the chillies, zucchini, tomatoes and spring onions and cook for a few minutes. Cut the fish fillet into cubes, add and let stand, but do not boil any more. Mix in the herbs and lemon zest and season with salt and lemon juice.