1.Steam potatoes in their skins (or gently boil) until nearly tender (if the potatoes are large, cut them in half first). Cool, peel and cube. Put half the potato cubes in a heavy-based, buttered, 2-litre casserole dish. Season with sea salt, freshly ground black pepper, a little paprika and half the dill.
2.Preheat oven to 180°C fan forced. Dust fish fillets with the seasoned flour. Heat oil in a large frying pan over a medium-high heat, then drop in 1 tablespoon butter. Add fish fillets, cook on one side until golden, turn and cook for 20-30 seconds, then transfer to a plate. Season fish with salt then transfer to casserole and add the rest of the dill.
3.Wipe out the pan, add remaining butter and when it is sizzling, add the mushrooms, cut side down. Brown quickly on both sides, stir in spring onions, cook for another minute, then tip into casserole on top of fish and season with a little salt and pepper. Top with remaining potatoes.
4.Mix eggs and cream together and pour over potatoes. Season with a little salt. Scatter cheddar cheese, parmesan and crumbs on top and dust with more paprika. Bake for 25 minutes until bubbling and crisp on top. Serve hot.