Chop up the onion and gently fry over a low heat for about 10 minutes. Add the chopped garlic and cook for a couple more minutes. Add the diced courgette and the chopped mushrooms. Cook over a medium heat until the mushrooms release their water. Add the fried tomato puree and cook for about 3 minutes so that the ingredients mix together. Strain the tinned tuna, add to the tomato-and-vegetable sauce and hey presto! you’ll have your filling ready. Boil the sheets of lasagne following the cooking instructions on the packet.
To make the béchamel sauce:
Melt the butter in a saucepan, add the flour and leave to cook for about five minutes. Slowly add the warm milk, stirring all the while. Add the salt and pepper and cook until it starts to thicken, which should take a few minutes. Arrange the lasagne in layers in an ovenproof dish as follows:
Start with a layer of béchamel sauce on the bottom, then a sheet of lasagne, followed by a layer of filling. Continue with alternating layers of sheets of lasagne and filling and then finish off by covering it all with béchamel sauce. Bake in the oven at about 180 ºC for about 30 minutes. Sprinkle some grated cheese on top to give it a gratin finish.