Fry the onion in olive oil for about 5 to 7 minutes on a medium-high heat.
Add the beef mince and garlic to the pan and continue to cook until the mince has browned. Once the mince has browned continue to cook for another 5 minutes.
Add the chopped tomatoes and their juices into the pan and mix well. Once mixed continue to cook for another 15 to 20 minutes, till thickened. You may not need to cook it for that long, keep an eye on it.
Add the stock cube, tomato puree, Worcester sauce, basil and herbes de Provence. Season with salt and pepper. Stir well. (If the mixture is still quite runny and watery, add 1 to 2 tablespoons of plain flour. This should thicken the mixture a lot.)
Now that the beef mince mixture has been cooked you can start on the white sauce. Melt butter in a large pan. Make sure to do this on a medium heat so that the butter doesn't burn.
When the butter has melted, add the plain flour and mix well until it forms a thick paste. When the paste has been formed, pour in some milk (amount unknown) while constantly mixing with a whisk. Do this on a medium heat, whisking constantly and continuing to add milk until the mixture is lump free and has thickened a lot.
Once the sauce has thickened you can add a handful of grated Cheddar cheese. I use Cheddar but I'm sure any other type of mature cheese will work well. Keep stirring on the heat until the cheese has melted and is smooth. Finally, season to taste with salt, then remove from heat.
To assemble the lasagne, start off by spreading half the meat mixture into the bottom of a baking dish. Layer the lasagne sheets over the top of the mixture and spread half the cheese sauce on top. Sprinkle with grated Parmesan cheese.
Repeat until all the mixtures have been used, ending with white sauce and a sprinkling of cheese.
Place in the oven for 30 minutes, keep an eye on it so it doesn't burn. Once it's bubbly and golden on top, and the pasta is tender, it's ready.