When I'm at a loss for what to have for lunch and don't want to cook, but don't just want a sandwich, this is my go-to lunch: a tuna melt bagel. It only needs three or four ingredients and a quick 15 minute blast in the oven.
Preheat the oven to 175C. To serve one, take a bagel and slice in half. Mix a tin of tuna with some salad cream or mayonnaise to taste, and spread on one half of the bagel. Spread cream cheese on the other side - I usually use two Laughing Cow light triangles but you could use any kind.
Sandwich the bagel together and wrap in foil. Place in the pre-heated oven for 15 mins.
Carefully unwrap, and enjoy. You probably need to eat this with a knife and fork rather than with your fingers - the cheese melts nicely (I think using the cheese spread is less overpowering than grated Cheddar) and the bagel softens but crisps a little on top. It's really good!