When Josh Jennings was planning his high school graduation party, in 2017, he asked his dad, Gary, to cook some barbecue for the crowd. Gary had dabbled in smoking meat with an offset he bought from a big-box store. He also had a 250-gallon propane tank he was slowly converting into an offset smoker, using plans from Franklin Barbecue: A Meat-Smoking Manifesto. With the party coming up, he had the motivation he needed to finish the project. His briskets earned high praise from guests, and Gary got a boost of confidence.He took on small catering jobs through word-of-mouth recommendations near his home, in Montgomery, northwest of Houston, and by 2021 he was serving to the public under a tent every Sunday. He bought a well-equipped…