Preheat your oven to 200ºc / 180ºc fan
Peel and quarter the potatoes. Cover them with water in a large saucepan, then bring to the boil and simmer for 15 to 20 minutes, until tender.
Whilst the potatoes are cooking, you can get going with the sauce. Melt the dairy free butter alternative in a large saucepan over a low heat. Stir in the flour, and cook for a couple of minutes, stirring regularly. Then, add the almond milk, bit by bit. (gently heating the almond milk before adding it can help stop lumps forming - see these tips for a perfect roux). Keep stirring until you have a smooth sauce. Bring the sauce to the boil, then simmer and cook for 5 minutes or so, stirring regularly, until the sauce is thickened. Take the sauce off the heat, then stir in the nutritional yeast and the mustard.
Get an ovenproof dish (I use a 20cm by 15cm dish for this). Spread the fish chunks over the base, making sure to double check for any bones. Then sprinkle over the peas or sweetcorn. Pour in the sauce, and give the whole thing a gentle stir.
When the potatoes are cooked, drain them well and mash until smooth with a splash of the almond milk and a knob of the dairy free butter.
Spoon the potatoes evenly over the top of the sauce mix, and run the prongs of a fork along the top - this helps to create lots of nice crunchy bits! Finally, lightly sprinkle the polenta over the top of the pie.
Bake in the oven for around 40 minutes. You want the top to be golden and crunchy, and the sauce to be bubbling up at the sides!