Heat a glug of oil in a frying pan over medium heat. Cook the mushrooms until browned, 5-7 mins. Add the onion and cook until soft, 3-5 mins, then add the garlic and half the sage. Cook for 1 minute more. Stir in the crème fraîche, stock powder, water (see ingredients list for amount) and cavolo nero. Stir to dissolve the stock powder. Simmer until the cavolo nero has softened, 4-5 more mins. TIP: Add a splash of water if the mix looks a little too dry!