It is your nature to avoid these conversations, and naturally you are upset that lunchtime has turned into boss-bashing time. When encountering an outrageous new boss, many employees need to share their experiences and vent. Since you do not wish to do so, you have other options to help make lunchtime conversations more palatable.
One approach is to come to lunch armed with other topics, whether work-related or not. The only item missing from your list should be the new boss.
If your co-workers return to blasting him, you can try to change the subject. At some point, they are going to ask why you do not join in the verbal slug fest, and you should tell them the truth, but not in a moralizing or conciliatory way.
Soon enough, your new boss is going to be repeating his outrageous behaviors. When this occurs, the lunchtime discussion will turn into reruns, and this opens the door for other topics. However, with a truly outrageous boss, there may still be lunches where he again becomes the topic du jour.
If this boss is as awful as you and your co-workers feel, the discussion in which you might want to participate is one that focuses on what to do about him.
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