"Alright cooky, you're going downtown!"

As you slap the cuffs onto Mac Doherty, the cook, you lay out your case:

Mac was a former criminal turned cook, hired by the late Mr. Kensington's affinity for employing reformed crooks.

The Butler was sick, so he asked Mac to help cover some of his duties, which on that particular day involved cleaning Mr. Kensington's guns.

Unfortunately for Mr. Kensington, the butter-fingered Mac discharged a pistol by mistake, striking Kensington through the heart, killing him.

Thankfully, the weed-eating going on in the garden masked the gunshot from most of the mansion occupants, giving Mac time to think.

Fearing a return to prison, Mac thought the best idea would be to frame the gunsmith, a fellow former inmate who knew his way around pistols.

Mac fetched a pistol from his suitcase and waited in the dining room for the gunsmith to arrive. 

He then fired a second shot, striking the poem above the fireplace. He locked away his gun, then ran down the hall and into the lounge, thereafter promptly calling the police.

Unfortunately, he failed to consider that had he been in the storage room as claimed, the stone walls would have prevented him from hearing the gunshot. The fact that there were no onions in the kitchen further discredited his story.

Much like the fabled onions, this case is cut and dry.

Well done!