since I already wrote, probably just add more.

Of course you do not want your boss to feel like that you take the "free" lunch for granted. For example, at the end of the lunch, you may something like "maybe this time it is my turn to pay." Almost always, your boss will say "no, no, bla blabla". You should not insist at this time! You can then say something like "Thank you for bla bla! That is great food/lunch, i really enjoy whatever dish, blablabla." Then everybody will be happy.

Again, just my 2 cents. There are likely many other better ideas from people here.