Slice down the fillet lengthways, but don't cut all the way through - you want to open out the fillet, like a book.
Put it under a piece of clingfilm and then bash it all over with a rolling pin or meat hammer, to flatten it a bit.
Remove clingfilm, then scatter the meat (cut side) with finely chopped garlic, zest of a lemon, chilli flakes, chopped sun dried tomatoes and some cooked, chopped spinach (2-3 squares of frozen stuff, defrosted would be perfect). Season with salt and pepper and any dried herbs you have to hand/like, then roll it up tightly.
Put it on a square of foil and drizzle with some olive oil and the juice of a lemon. Wrap it up tightly and bake in the oven for about 20 minutes.
Don't overcook it, as this is what makes it rubbery. Pork fillet is a cut that you can eat slightly pink, so it is better under than overdone. (Only very slightly pink, though!)