Preheat the oven to 180ºC.
Spray a saucepan with low calorie cooking spray and sauté the onions over a medium heat for 4 minutes until softened.
Add the garlic granules and Italian herbs and continue cooking for another minute.
Dissolve the stock cube in the boiling water. Stir in the tomatoes and stock and bring to the boil. Reduce the heat and allow to simmer for 15 minutes while you prepare the cannelloni.
Spray a frying pan with low calorie cooking spray and sauté the leeks for 5 minutes over a medium heat, until soft, then place in a mixing bowl and allow to cool for 5 minutes.
Mix the cream cheese, ham, mustard and garlic granules into the cooled leeks, and season with salt and pepper.
Fill the cannelloni tubes with the ham and leek mix, using a blunt knife to push the filling right down.
Pour half the sauce into the ovenproof dish and arrange the filled cannelloni on top. Pour the remaining sauce on top, ensuring the pasta is fully covered.
Tear the mozzarella into small pieces and scatter over the top.
Bake in the oven for 35-40 minutes until the pasta is tender.
Serve with a crisp mixed salad.