Spray a large saucepan with low calorie cooking spray and brown the lamb for 5-6 minutes. Slice the carrots, chop the green beans and grate the courgette, and add to the saucepan along with the onion and garlic.
Add the tomato puree, lamb stock pot and beef stock cubes. Cover with boiling water (around 900ml, depending on the size of your saucepan), mix thoroughly and pop a lid on. Cook for 25 minutes on a low heat, stirring occasionally to ensure it doesn't stick to the bottom.
Pour the meat into a baking dish, top with the mashed potatoes, spray with low calorie cooking spray and place in the oven for 30 minutes until golden and crisp!