Banerjee, who was onway to Kharagpur in West Midnapore district, stopped her convoy and went to Jadu Babu's Market at Bhowanipore close to her Kaligrhat residence.
She first spoke to a number of retail sellers.
"Didi ('elder sister' as Banerjee is affectionately called) asked me at what price I was selling onions. When I said I am selling the bulb around Rs 130 and Rs 140 a kg, she asked me at what price I was buying it. I informed her I was buying it at Rs 120 a kg from the stockist," said a seller Shila Roy.
Banerjee then asked her to name the stockist.
On knowing his name, Banerjee summoned him and asked him at what price he was selling it to the retailers. After the stockist said he was getting a price of Rs 120 a kilo, the Chief Minister wanted to know why the rate was so high when the state government was selling the vegetable at Rs 59 a kilo.
"Dal me kuch kala hai (something is wrong)," Banerjee said.
She then spoke to other onion sellers in the market.
In the wake of the devastating blaze that took the lives of 43 workers at a bag factory in Delhi, Banerjee instructed the market authorities to ensure that plastic sheets being used atop the stalls be removed.
Sometime after Banerjee left the market, police officers arrived there and started preparing a list of those trading in onions.
( With inputs from IANS )