Preheat the oven to 200c fan.
Cut the potatoes in ½ or ¼s. In a large pot of water, add the potatoes fill up with water to cover, season with salt, some rosemary and a few smashed garlic cloves and bring to a simmer. Cool until tender and a knife goes through.
As the potatoes cook, In a pan sweat the leek and garlic with a pinch of salt and black pepper until soft. Sprinkle in 1 tbsp flour, mix and start to slowly whisk in the milk until all combined. Bring to a simmer to thicken, add the stock cube, mustard and simmer for a few minutes before adding in a handful of spinach. Mix to wilt then turn off the heat. Finish with some lemon juice. Dice the salmon and fold into the sauce. Tip into a suitable baking dish or bake in the same pan if oven safe.
Drain the potatoes, tip into a bowl or back into the pot, add 30g grated cheddar, 1 tbsp olive oil, the chopped spring onions and parsley, salt and pepper. Using a spoon roughly mash leaving nice chunky pieces. Top the salmon mix with the mash. Add the remaining 10g of cheese and bake for 20-25 mins until golden brown.
You can finish under the grill the make extra golden and crisp.
