White sauces aren't too tricky... they just take a bit of watching to make sure they don't go lumpy. (But then again, isn't that what the blender is for? )

You could probably substitute with a tin of condensed soup... cream of mushroom or something similar. You may need to add in a tiny bit of water (or milk/cream) so that it's not too thick, but not so much that it's 'soup' consistency. Perhaps pop it in the pot with the drained pasta and tuna for a few moments to heat it through. Then add your toppings and throw it into the oven! Good luck.