Dry the lamb fillet and season all over. Heat a little oil in a small frying pan and put in the fillet, fat-side down at first, then turning, for 2 minutes in total. Remove to a small roasting tin or pan lined with foil (for easy clean-up) and dust all over with the sumac. The fillet is now ready to cook.
For the saffron dauphinois, heat the butter in a small frying pan and add the onion and seasoning. Fry slowly and gently for 15 minutes, till golden and frizzled. Meanwhile, bring the cream, saffron and nutmeg just to the boil in a medium saucepan, then turn off the heat, season and cover.
Slice the potatoes as thinly as you can (I recommend a mandoline) and add to the cream. Bring the mixture to a simmer, stirring constantly with a rubber spatula to stop the mixture sticking. Butter a shallow, ovenproof dish about 19cm square, with a 300ml capacity – and layer in half the potatoes, followed by all the onion and half the cheese. Add the remaining potatoes, top with the cheese and a few crumbs and bake at 180°C/Fan 160°C /Gas 3 for 30–40 minutes. Check the potatoes are fully cooked and tender by poking with a knife in 3 or 4 different spots. Once done, set aside to cool slightly.
Roast your lamb at 200°C/ Fan 180°C/ Gas 4 for 12–15 minutes. Check doneness by poking with a knife, or if you have a digital thermometer, it should read 54°C (for rare) or 60°C (medium). Fold the foil up around the meat loosely and leave to rest for 5–10 minutes before carving into slices and serving.
To make the mint and sumac butter, if using, mix all the ingredients with salt and pepper, spoon into a small pot or bowl and put in the fridge. Add a spoonful on each portion of lamb when serving.
To make the leek straw, if using, cut off a 4cm section of leek, roughly where the white meets the green. Slice as thinly as you can into matchsticks. (If you wish, boil the remaining leek and serve as a side vegetable.) Shortly before serving, heat the oil in a frying pan, add the leek and cook until golden (the colour of straw). Remove immediately to a piece of kitchen paper towel, before sprinkling with salt and serving on top of the lamb and butter.