Heat oven to 200C/180C fan/gas 6. Cook the potatoes in a pan of boiling water for 10 mins, with the leeks in a covered steamer over the pan. Remove the steamer, add the peas to the potatoes and cook for 10 mins more. Drain the peas and potatoes, then mash with ½ tbsp crème fraîche, the lemon zest and juice, dill and bouillon.
Arrange the leeks in a shallow ovenproof pie dish (about 18 x 24cm). Add the tomatoes, cod and prawns, then dot over the rest of the crème fraîche. Spoon over the mash, then spread it lightly to the edges with a fork.
Bake for 30-35 mins until bubbling round the sides of the dish. Serve with veg, if you like. If you make this ahead and are cooking from cold, bake for about 10 mins longer.