Peel and finely dice the onions. Scrub new carrots, peel old ones, then finely dice. Grate the cheese finely for the cheese sauce.
Lightly fry the onions, carrot and beef mince in a saucepan. Just cover the meat in the pan with tomato based sauce and dried herbs. Cover and cook until tender, about 30 minutes, then remove the lid, adjust consistency if necessary.
To make the cheese sauce, melt the margarine in a saucepan then add the flour, and cook, stirring, until sandy in texture and colour. Add the milk in one go and whisk vigorously so that no lumps develop. Cook for about 5 minutes. Add the mustard and nearly all the cheese, reserving a little for the top, and allow to melt. If too thick, add a little more milk.
Put a layer of meat sauce in the base of the tin, and cover with Lasagne sheets. Repeat this twice. Pour the cheese sauce over the final layer of Lasagne, and sprinkle with the reserved cheese.
Put into the preheated oven and bake until the pasta is tender and the top is brown, about 30–35 minutes.