Peel the potatoes and slice thinly, then parboil in a pan of boiling water for about 5-10 minutes until soft but not breaking up.
Put 1 tbsp olive oil and 15g butter in a large pan.
When the butter begins to sizzle, add the leeks to the pan.
Sauté the leeks gently until they are soft and slightly golden.
Transfer them to an ovenproof baking dish and season.
Put the other 1 tbsp of olive oil and 15g butter into the pan.
When the butter begins to sizzle, add the whole bag of spinach leaves.
Cover the pan and cook gently for approximately 5 minutes, until the spinach has wilted.
Place the wilted spinach over the leeks in the ovenproof dish and season.
Place the haddock pieces on top of the spinach and season.
Put the cornflour into a bowl and add the milk, then mix together. The cornflour will help prevent the crème fraîche from splitting when the dish is baked.
Add the Dijon mustard to the milk and cornflour and mix thoroughly.
Add the crème fraîche and the two cheeses and mix together.
Spread the mixture over the fish and vegetable layers.
Top the ovenproof dish with the potatoes.
Melt the remaining butter in a pan and brush over the potato slices.
Bake in the oven for 30 minutes, until the sauce is hot and bubbling and the potatoes are golden brown.