Preheat the oven to 425°F (200°C). 

Start with the celery root mash topping since it will take the longest. Bring a large saucepan of water to a boil and add the celery root chunks. Cook until completely softened, then drain in a colander. Return the celery root to the pan and add the sour cream, salt, and pepper. Mash well, then add the butter to melt in. Set aside.

Meanwhile, season the fish with salt and pepper and bake in the oven for 12 minutes. Remove and leave to cool for a few minutes, then break the pieces up into large chunks. Set aside. Remove the parchment paper from the fish’s baking sheet and place 4 greased ramekins or miniature casserole pots onto it.

Heat the olive oil in a large non-stick pan or wok and add the garlic. Cook over low heat for a minute to soften, then tip in all the spinach. Increase the heat to high and stir continuously until the spinach completely wilts. Use your spatula or wooden spoon to ‘chop’ into the spinach and break it up as it wilts.

Add the prawns and cook for another minute or two. They don’t take long: once pink and no longer grey, they are done. Squeeze in the lemon juice and allow it to evaporate.

Add the cream, sour cream, and most of the chopped tarragon. Leave the mixture to bubble down to a thick mixture that is not too watery. Tip in the cooked white fish chunks and season with salt and pepper. Fold gently to combine and be careful of breaking up the fish too much.

Divide equal amounts of the creamy fish mixture between the 4 pots and cover with the celery root mash. Scatter over the Parmesan and bake in the oven for 12-13 minutes to melt the cheese and warm the pies through completely.

Garnish with the remaining chopped tarragon and serve with a lovely, simple salad on the side (not included in the macros).