Heat oven to 180C/160C fan/gas 4. Pierce the potatoes with a sharp knife and bake for around 1 hr until a skewer slides through without resistance. Leave to cool, then peel and mash into a bowl. Add the parmesan and lightly mix. Season with plenty of salt, pepper and a little freshly grated nutmeg.
Put the watercress in a sieve and pour a kettle of boiling water over it. Allow to cool for a couple of mins, then squeeze out any excess moisture and blitz in a blender. Once cool, mix with the potatoes, along with the ricotta and egg yolks and sieve in the flour. Mix lightly with your hands to form a dough. Roll the dough, a quarter at a time, into long thin sausages on a floured surface, and use a sharp knife to cut a few lines widthways on each one.
Bring a pan of salted water to the boil. Heat the olive oil in a large non-stick frying pan. Add the thyme and radishes and fry for about 5-8 mins until coloured. Tip half the gnocchi into the salted water and boil until they float to the top (about 3 mins). Fish them out carefully with a slotted spoon and put them on a plate. Repeat with the remaining gnocchi.
Add a knob of butter to the radish pan and tip in the boiled gnocchi. Fry until starting to colour, about 3 mins. Season, add the lemon zest and squeeze over the lemon juice, and grate in a little parmesan. Divide between plates and top with more parmesan and a good grating of black pepper.