A Sandwich

(Sidney Paget, Dressed like a common loafer. Credit for Internet Archive)

'I only wish that you could come with me, Watson, but I fear that it won't do. I may be on the trail in this matter, or I may be following a will-o'-the wisp, but I shall soon know which it is. I hope that I may be back in a few hours.'
He cut a slice of beef from the joint upon the sideboard, sandwiched it between two rounds of bread, and, thrusting this rude meal into his pocket, he started off upon his expedition.
("The Beryl Coronet", 321)

This conversation and narration are different from the description of food which I already wrote about. In this scene, Holmes disguised and was going to go to investigate the case. He might know something about the case but it is still investigating at that time. It seems that Doyle uses Holmes's "rude meal" to emphasise his personality. Holmes's disguise is perfect as Watson says that "[h]e hurried to his chamber, and was down again in a few minutes dressed as a common loafer. with his collar turned up, his shiny seedy coat, his red cravat, and his worn boots, he was a perfect sample of the class" (321). Therefore, he seems to be not clumsy. He had a few minutes to do something, and he chose to disguise perfectly not cooking a nice meal. Or we can think in the other way. His disguise to lower class including "the rude meal". In both ways, we can see Holmes's priority. His priority is solving cases not eating a good meal.