When the sides met at Old Trafford two weeks ago Onions bowled the last over with 12 needed and Steven Croft hit the last ball for six to clinch Lancashire’s win.
This time they needed 11 with two wickets standing, but Onions conceded only four and had Simon Marshall caught at third man.
The catcher was Callum Thorp, who had opened up with eight overs for eight runs and returned to take 1-6 in his two overs at the death. He took his only wicket with the last ball of the 49th over, which Kyle Hogg edged to Phil Mustard after making 15 off 16 balls.
Durham’s total of 205 for eight after being put in had looked inadequate, despite an unbeaten 42 off 37 balls by Garry Park, which included two sixes off Sajid Mahmood as 20 came off the final over.
They were again indebted to skipper Dale Benkenstein, who went in with the score on 28 for three and made 64 off 108 balls.
Despite the slowness of the pitch, he would have hoped for a bigger total against an attack featuring 20-year-old seamer Steven Cheetham for the first time in the absence of Jimmy Anderson, Andrew Flintoff, Glen Chapple and Dominic Cork.
Park later showed electrifying pace as he swooped from point to run out Lancashire captain Stuart Law, but Mal Loye looked like steering the visitors home.
After being dropped on 32 by Gareth Breese at slip off Steve Harmison he went on to make 77 and put on 75 for the fifth wicket with Croft.
Croft drove Harmison high over long-on for six as the game began to slip away from Durham with ten overs left, but then Loye strained a hamstring when he narrowly avoided being run out. After calling for a runner he had an unnecessary heave at Harmison and skied a catch to extra cover.
Croft then fell in Harmison’s final over, edging to Mustard, to leave 36 needed off five overs. With Thorp, the Australian seamer, conceding only three off each of the 47th and 49th, Onions was left to complete the task.
With two group games left Durham have now won three and lost three, while Lancashire are a point ahead of them courtesy of a no-result.