Preheat the oven to 180C/350F/Gas 4.
Lightly score or prick the potatoes. Spread out the salt on a baking tray to make a bed of salt and sit the potatoes on top. Bake in the oven for 45 minutes to one hour, or until tender.
Allow the potatoes to cool until comfortable enough to handle. Cut in half and spoon the cooked potato out of the skins.
Finely mash the potato or put through a potato ricer. Add the flour, parmesan and egg yolk. Season with salt and pepper and mix well. Keep mixing until the potato forms a dough.
Working on a lightly floured surface, divide the dough into four. Roll each quarter into a sausage shape about 20cm/8in long and cut into 2cm/1in pieces. Shake off any excess flour.
Melt the butter in a frying pan and lightly fry the breadcrumbs until golden-brown. Tip into a small bowl and set aside.
Heat a tablespoon of oil in the same frying pan and fry the gnocchi in batches until golden-brown on all sides. Remove from the pan, set aside and keep warm until all the gnocchi is fried.
Remove the skins from the sausages, break up the sausagemeat and fry with a little oil for four minutes, then add the onion and fry for a further three minutes until it is all golden-brown. Add the garlic and fry for another minute.
Add the stock, peas, cream, capers and parsley. Bubble over a high heat to reduce the volume of liquid slightly and thicken the sauce. Finally add the fried gnocchi and warm through in the sauce for a minute or two.
To serve, divide the lettuce between four plates or bowls, serve the gnocchi on top, scatter with the fried breadcrumbs, grated parmesan and finely grated lemon zest.