Can you recount the story from Chapter 23 involving the Yogi who had doubts about Sai Baba?

Certainly. Chapter 23 narrates an incident where a practitioner of Yoga (Yogabhyasi), who was well-versed in the Yoga Shastras but unable to achieve Samadhi, came to see Sai Baba with Nanasaheb Chandorkar. He hoped the 'King of Yogis' could resolve his difficulties. However, upon arrival, he saw Baba eating stale bread with raw onions, which caused a strong doubt to arise in his mind about Baba's spiritual greatness. As the text of Chapter 23 explains, the omniscient Sai Maharaj immediately addressed this unspoken thought by remarking to Nana, 'only those whose digestion is strong should eat onion.' This profound insight startled the Yogi, dissolving his doubt. He then surrendered with true devotion, listened to Baba's teachings, and returned with a happy mind, his spiritual path clarified.