Begin by removing the hard ends of the Brussels sprouts and then tear off a few of the good leaves too, keep these to one side and as these will turn extra crispy once cooked.
You can either boil or steam the sprouts for 4-5 minutes to part cook them or just add to a large frying pan with oil and seasoning and allow to cook with a lid on for 6-8 minutes until slightly soft and golden. You want to make sure you keep them fresh and crunchy.
Remove the sprouts and set to one side and add another glug of oil. Allow to heat and add in the oyster mushrooms, cook over a high heat until crisp and golden then add in your spoonful of marmite and stir so all mushrooms are evenly coated before adding some freshly ground black pepper to taste. You can add in more marmite or seasoning depending on your preference.
Next make a little space in the side of your frying pan and add in the sage leaves, you may need an additional small glug of oil depending on how much you added initially. Allow the leaves to cook and turn crisp, this should only take a minute or so then remove and set to one side to garnish.
Add the sprouts back into the pan and gently mix with the mushrooms. Cook for a further minute or so to reheat the sprouts then serve and sprinkle over the sage leaves.