Wash and roughly chop the peppers, tomatoes and mushrooms. You don't need to chop them too finely, as we will be blending them later. Place in a saucepan and put on a low heat with a lid on the pan.
Meanwhile, fry the mince in the oil until browned. Finely chop the garlic and add to the meat. Then crumble in the stock cube, add the tomato puree, and the Italian seasoning. Season with a little salt and pepper, and set aside.
Check to see if the veg is soft. Once it is, blend it with a handheld blender. Take care, as it will be quite hot and you don't want to splash yourself, so if your blender tends to splash you can wait for the veg to cool first. You could use a normal blender if you want, but I find it ends up being more to wash up and a lot more effort.
Once the veg is blended into a sauce, add to the cooked mince and mix together. If you are using different quantities, and have excess veg sauce, you can freeze the extra. Set the mixture aside.
Now make the white sauce. Melt the butter in a pan on a low heat. Once melted, turn off the heat and add the flour. Mix thoroughly to make sure there are no lumps. Next add the milk, a little bit at a time, mixing in between to make sure that each time to ensure it is fully combined. Once all the milk is added, turn the heat on medium and mix constantly until the sauce has thickened, around 7-10 minutes. This may take a little while, but just keep mixing until it is thick.
Now assemble the Lasagne. Spoon about half of the meat mixture into an oven-safe dish. Cover the mix with one layer of lasagne sheets. Break them into pieces if they don't exactly fit the shape of your dish, but don't worry if there are a few small gaps. Then spoon half of the white sauce on top and cover again with lasagne. Repeat those steps with the other half of the meat and finish with the last layer of white sauce.
Cook in the oven on 180°C for 25 minutes. Add a layer of grated cheese on top and cook for a further 5 minutes, until fully melted.
Cut into 6 slices and plate with a fish slice - the first piece might fall apart a little but it will taste delicious anyway - and enjoy!