From late 2018 through 2020, Nomon Tim Burnett gave talks on all 48 of the koans (Zen stories) in the Mumonkan ("Gateless Barrier") koan collection. Talks by other sangha teachers on these stories are also listed. The Mumonkan is the most widely studied koan collection in Japanese Zen. 

You might wish to listen first to this overview talk on koans given to Zen Buddhism Ireland in September, 2020:  Nomon Tim Burnett : What are koans?