Melt the butter in a large frying pan over medium heat. Add the onion and sliced mushrooms and cook for 3-4 minutes. Add the risotto and stir well. Add a small amount of vegetable stock, keeping the pan on a medium heat. When the stock has been absorbed, add a little more vegetable stock and continue to stir. Repeat this process for 8-10 minutes, then taste the risotto. If cooked, stop adding stock. The risotto will now have a soft creamy texture.
Season with salt and pepper. Add 3/4 of the cubed Cheddar cheese, tomato and basil leaves, stirring over heat for 2-3 minutes until the cheese melts and the basil wilts.
Serve and top with fresh basil and diced cheese.