In a pot melt butter or margarine and stir in flour until you have a thick paste. Place on very low heat. I remove mine from the heat altogether as I cook with gas.
Blend in you warm milk a little at a time, until your paste is liquid, stir continuously while adding the milk, so that no lumps form. Return to stove or turn the heat up, add your salt and mustard and ½ your cheese. Keep stirring till all the ingredients are mixed in and simmer until your sauce thickens. Remove from heat. You can leave out the cheese and mustard powder if you want a plain white sauce.
In your lasagne dish spoon a thin layer of cheese sauce, then a layer of lasagne sheets, avoid overlapping.
Next, spoon over a layer of tuna mixture. Cover with a layer of lasagne pasta. Spoon over a layer of cheese sauce. Repeat ending with the cheese sauce on top. The smaller your lasagne dish the more layers you will get. Sprinkle the remaining cheese and place in the oven for about 15 to 20 minutes.
I sometimes like to add vegetables to the tuna and pasta sauce, frozen peas and carrots and corn, to bulk it up. I have big eaters and this recipe never has left overs! I also sometimes sprinkle Parmesan cheese on top.