1. Heat your oven to 180C/Gas Mark 4. Grease a medium-large baking dish with butter.
2. Make the mushroom filling first – heat the butter in a frying pan and once it starts to bubble, add the mushrooms and turn up the heat. Season with salt and pepper and cook for a couple of minutes. Add the garlic and cook for another minute. Stir through the chopped parsley and leave to one side to cool.
3. To make the tomato sauce, heat the olive oil in a pan. Throw in the garlic and fry for 5 seconds before pouring in the chopped tomatoes (be careful not to let the garlic burn). Stir and bring to the boil. Add the sugar and some seasoning then simmer for 20 minutes until it becomes thick.
4. While you are waiting for your tomato sauce to thicken, start on the béchamel sauce – melt the butter in a pan then stir in the flour. Leave to bubble for a minute or two before pouring in some of the milk. Whisk until it starts to thicken, then add some more. Continue until you have used all the milk and you have a thick white sauce. Turn off the heat and stir in the grated nutmeg, mustard and some seasoning to taste.
5. Pour the tomato sauce into the bottom of the greased baking dish. Spread so the base is covered.
6. Fill the cannelloni tubes with the mushrooms and arrange on top of the tomato sauce.
7. Pour over the béchamel sauce and top with the grated parmesan. Cook in the oven for 50 minutes.