Preheat the oven to 220C, fan-forced. Line 2 oven trays with baking paper. Thinly slice the unpeeled potato and spread in a single layer on the lined trays. Melt 20g butter in a medium saucepan over medium heat. Brush the potato with butter and season with salt. Reserve the pan. Bake the potato for 20 mins or until tender.
Step 2 — Start prep
While the potato is baking, rinse the reserved pan, fill three-quarters full with water and bring to the boil for the vegetables. Trim the beans and cut into 2cm lengths. Finely chop the onion. Finely grate the zest of half the lemon*, then juice the half. Finely chop the dill, including the stems. Finely grate the parmesan. Cut the fish into 1.5cm chunks.
Step 3 — Finish prep
Cook the beans in the pan of boiling water for 2 mins. Add the peas, return to the boil and cook for a further 1 min or until the beans and peas are tender. Drain. Reserve the pan. Crumble 1 stock cube* into a heatproof jug, add 250ml (1 cup) boiling water and stir to dissolve.
Step 4 — Make sauce
Melt 20g butter in the reserved pan over medium heat. Cook the onion, stirring regularly, for 4-5 mins until softened. Add 1½ tbs flour and cook, stirring, for 1 min or until golden. Gradually add the stock, stirring constantly to prevent lumps forming. Add the lemon zest, 1 tsp lemon juice* and 1 tsp dijon mustard and bring to the boil, stirring until thickened.
Step 5 — Finish pie filling
Stir the fish into the sauce and cook, stirring occasionally, for 2-3 mins until almost cooked through. Stir in the beans, peas and dill. Taste, then season with salt and pepper.
Step 6 — Get ready to serve
Divide the pie filling among 2 x 375ml (1½ cup) ovenproof dishes. Overlap the potato slices to create the topping and scatter over the parmesan. Bake the pies for 10 mins or until browned. Take the pot pies to the table and serve.