Cut the pork tenderloin into 8-10 medallions then season with salt & pepper.
Cut the onion and pepper into strips. Roughly quarter or halve the mushrooms according to their size.
Heat the oil a frying pan, then brown the medallions for a couple of minutes on each side, then transfer them to a plate to rest.
Remove the fat from the pan, add the oil and the onion, red pepper and mushroom pieces. Add the garlic and sage, then fry for about 5 minutes until the vegetables soften.
Pour in the stock and bring to the boil.
Mix the cornflour with 2 Tbsp water, stir this into the mushroom sauce. Add the medallions and cook over medium heats for about 5 minutes.
When the sauce has thickened, and pork is cooked through, turn off the heat.
Season to taste and serve with the chopped spring onions scattered on top.