Pre-heat the oven to 180ºC.
Spray a large saucepan with low calorie cooking spray and over a medium heat, sauté the onions, carrots, celery and mushrooms for 10 minutes, until they begin to soften.
Add the mustard powder, garlic granules, thyme and tomato puree to the pan and stir well.
Stir in the stock, balsamic vinegar and soy sauce and bring to a simmer. Allow to cook for 10 minutes, until the vegetables are just soft.
Meanwhile place the sliced potatoes into a pan of cold salted water and bring to the boil, reduce the heat and simmer for 3-4 minutes. Drain.
When the vegetables are just soft, stir in the lentils, and pour the mix into an oven proof dish.
Arrange the par cooked, sliced potatoes on top, spray with low calorie cooking spray and sprinkle on a little salt and freshly ground black pepper to taste.
Place in the oven for about 30 minutes, until the potatoes are crisp and golden, but soft on the inside.
Serve with your choice of accompaniment.