Start by heating the oven to 170`c, gas mark 5. Heat the olive oil in a large frying pan and sauté the onion for 4 minutes until soft. Add the minced beef and sliced mushrooms to the pan and cook for just 2 minutes to brown the meat. Remove off the heat and stir in the chopped tomatoes, 450ml water, the contents of the `cheesy Bolognese pasta bake` sachet, fusilli pasta, mozzarella balls and half of the grated cheese. Tip the mixture into an oven-proof dish and even out, making sure that the pasta is submerged into the juices. Sprinkle over the remaining grated cheese and bake in the oven for 30-40 minutes, or until the top is golden brown. Remove from the oven and serve immediately.