What is a good example of Atticus seeing inherent goodness in Mr. Underwood?
I don't know if there's a specific example of Atticus seeing the good in Mr. Underwood, but there are at least two instances where Mr. Underwood proves himself a good man. The first is in chapter 15, when Atticus is guarding Tom at the jail. A mob appears, seemingly bent on pulling Tom from the jail and lynching him. Scout and Jem arrive, and Scout's innocence diffuses the situation, but they find out afterward that Mr. Underwood had a gun on the crowd the entire time, and would have fired if they gave him reason. Later, Atticus remarks that Mr. Underwood's behavior was unexpected, as he "despises Negroes, can't stand to have one around him."
After Tom's death, Mr. Underwood writes a scathing editorial in the paper. Scout describes it thus:
Mr. B. B. Underwood was at his most bitter, and he couldn't have cared less who canceled advertising and subscriptions. (But Maycomb didn't play that way: Mr. Underwood could holler till he sweated and write whatever he wanted to, he'd still get his advertising and subscriptions. If he wanted to make a fool of himself in his paper that was his business.) Mr. Underwood didn't talk about miscarriages of justice, he was writing so children could understand. Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom's death to the senseless slaughter of songbirds by hunters and children, and Maycomb thought he was trying to write an editorial poetical enough to be reprinted in The Montgomery Advertiser.
Here the mockingbird symbol is present again. Lee is using the character of Mr. Underwood to hgihlight Tom's innocence and the gross injustice that led to his death.