This joke is a play on words. The quitting sandwich shop owner is saying that only the customer can make themselves a sandwich by choosing the ingredients and creating their own "hero" sandwich. However, the phrase "make yourself a hero" can also be interpreted as empowering oneself and taking control of one's own destiny.
oh. (chuckle)
I'm glad I could help clarify the joke for you! If you have any more questions or need further explanations, feel free to ask.
