Heat the oven to 200°C/Fan 180°C/Gas 6. Bring a large pan of water to the boil. Pull the leaves off the cauliflower and discard. Roughly chop the cauliflower. Add to the boiling water and simmer for 10 mins till just softened.
While the cauli simmers, trim the top and bottom off the leek. Rinse out any grit and slice into 1 cm rounds. Halve the tomatoes.
Warm a frying pan over a medium heat. Add the leeks and tomatoes to the pan with 1 tbsp oil. Cook and stir for 5 mins till the leek is soft and the tomatoes are juicy. Season with a pinch of salt and pepper, then turn off the heat.
Pour the crème fraîche into a bowl, keeping back 1 tbsp for later. Sprinkle in the saffron powder. Zest and juice half the lemon and add to the bowl. Season and stir together. Add the fish pie mix and stir.
The cauliflower should be cooked by now. Drain, then tip back into the pan. Set over a low heat and mash – this will cook off any excess moisture. Add the reserved 1 tbsp crème fraîche. For a smoother mash, blitz in a food processor. Season generously.
Spoon the saffron fish mix into a small baking dish. Top with the leeks and tomatoes. Spoon over the cauliflower mash. Crack over some black pepper.
Bake in the oven for 25 mins till the top is golden and crunchy and the fish is cooked through.
While the fish pie cooking, finely shred the cabbage half. Pop into a pan and cover with boiling water. Simmer for 5 mins till bright green. Drain. Serve the fish pie with the cabbage and the rest of the lemon sliced into wedges for squeezing.