There is a professor who has made several reference letters for me, and I would like to thank him. However, I have some doubts as to whether it would at all be appropriate to thank him with dinner delivery to his office, say. I feel that simple verbal thanking is not sufficient at this point, but then, the atmosphere of formality is still there at the level of university relationships. What are one's ideas on this? If sending him a fine dinner would not be appropriate, especially without first discussing this with him, what is another way of thanking a professor, which would be considered "formally appropriate"?
I would avoid giving food-- especially a full meal-- to a professor, unless you know them very well. This is because you may not know their tastes or dietary requirements; for instance, unwittingly giving them a dish with nuts in if they have a severe allergy is definitely not a good idea.
It's also a bit strange to deliver such a meal to their office. They probably want to go home and eat their dinner there, rather than sit in the office and eat your gift.
While you are understandably grateful for the reference letters, the professor probably just considers this part of their job. If you really want, a nice handwritten card saying thank you would certainly suffice.