Begin by butterflying the pork loin. With the joint fat side up, using your sharpest knife slice horizontally along one side, so it can be opened like a book. Season the pork liberally with salt and pepper then make the stuffing.
In a food processor combine the stuffing ingredients. Add enough oil to form a paste. Spread this paste along the middle of the open pork. Roll the pork up and tie around the joint with string. Place in a sturdy roasting tray, drizzle with oil and put in the oven.
After 15 minutes turn the oven down to 150C. Continue cooking until the pork is at least 55C when tested with a probe thermometer. If eating now, allow to cool for at least 15 minutes before serving. Otherwise, allow to cool and refrigerate until needed.
To pickle the radishes, sprinkle salt and sugar into white wine vinegar until it tastes quite sweet. Add the radishes thinly sliced and leave to macerate while you do everything else.
Now make the parmesan cream. For best results whizz up the parmesan in a food processor to make small rubbly pieces of cheese - they melt unevenly and make a pleasing texture. Gently warm the cream with half the cheese but don't boil, you just want the cream to melt the parmesan.
Lay the spinach in warm serving bowls. Drizzle over some of the parmesan cream so that it wilts. Add a thick slice of porchetta, garnish with radish slices and a little more cream. Scatter over the remaining parmesan and drizzle with a touch of balsamic vinegar if you like.
Notes
Always test roasting joints with a probe thermometer. They're dead cheap and the safest, surest way to prevent over or undercooking. Buy one from Amazon.