No need to preheat oven.
Place topside beef, mushrooms and water into the roasting bag and season with dry KNORR mixture. Seal at end with blue tie supplied.
Very gently massage KNORR mixture into the beef on a stable surface. Ensure dry mixture is spread evenly.
Place bag sideways in a cool oven dish, ensuring ingredients are evenly spaced.
Bake at 180°C for 1 hour and 40 minutes on the lowest shelf in the oven. Ensure that the grill is off at all times. Keep enough room for the bag to expand (bag must not touch the sides of the oven).
Cut the bag open and transfer to a serving dish. Spoon the sauce over your beef roast.