Stir in the tomato puree, Oxo cubes and tinned tomatoes. Add 200ml water and stir well.
Put the lid on the pan and simmer for 20-30 minutes.
Adjust seasoning if necessary.
Spread a layer of the mince mixture on the bottom of a decent sized oven proof dish, then add a layer of lasagne sheets, then repeat until you reach the top of the dish, ending with a layer of lasagne sheets. Make sure you leave enough room for the sauce.
Mix the yogurt, eggs, mustard powder and salt to taste. Beat well to make sure it's mixed smooth.
Cook in the oven for 25-30 minutes, on 190-200°C.