The day before, marinate your pork tenderloin in the blackberries. Smash up a few and smear it all over the pork then stick it in the fridge.
Preheat the over to 200*C.
In an oiled baking tray, lay your pork, cover with the blackberries and marinade and then scatter the roughly chopped onion around.
In another tray, cut your broccoli into small florets, throw about a tablespoon or so of olive oil on it and some sea salt and black pepper.
Stick both trays in the oven and bake for about 30 minutes.
After 30 minutes, take the broccoli out, stir it about a bit, then scatter the chopped up feta over the broccoli and put back in the oven.
Take the pork out of the oven and put the loin on a chopping board to rest while you make the sauce.
Stick your baking pan on the hob on low, pour in 1 tablespoon of balsamic vinegar and scrap all the lovely onions, blackberries and pork juice about. Then add in your 4 tablespoons of stock and bring to a little boil.
Then cut the pork into thick slices, and spoon some sauce on top.
Serve with the Broccoli and Feta.