Middlesex 188 (Onions 6-45) and 179 (Onions 4-28) drew with Durham 238 and 82 for 6 (Finn 4-43)
Four wickets from Steven Finn gave Middlesex a glimpse of an unlikely victory on the final evening but after more than half a match of rain interruptions the outcome was the one that was always the easiest to predict.
An original target of 130 to win from 45 overs favoured a Durham win but heavy rain in mid-afternoon left them with a much more testing 122 off 16. While they might have backed themselves to score that in Twenty20 conditions, against Finn and Tim Murtagh coming off a full run to a cordon of slips and gullies, it was a different proposition.
In the event, Middlesex came closest to pulling off a result after Finn, having rubbed off a little rustiness in the first innings, had worked up a good pace and claimed four for 43, Durham finishing six down.
It meant the 23-year-old could walk away from Lord's with a spring in his step despite the challenge thrown down by Graham Onions, whose ten wickets in the contest gave notice that he is pushing strongly for a recall to the England Test side.
"I got better as the game went on. I had not bowled since the last Test in Sri Lanka so there a few cobwebs to blow away and I felt I bowled pretty well today," Finn said.
"It is great to have that competition, too. Graham was the best bowler in the match by a long way, he bowled beautifully.
"He and I are in a similar position, on the fringes of the Test team. We are all fighting for the same spots but no one grudges anyone else doing well and I have huge respect for Graham, he is an awesome bowler."
In the final, shortened contest, Middlesex fancied themselves to steal a win, Finn said, given the conditions and the dilemma Durham faced over whether to chase the runs or content themselves with a draw.
Yet they had been saved in effect by the ninth-wicket partnership between Tim Murtagh and John Simpson that held up Durham in an afternoon that had started with Onions, bristling with confidence after twice accounting for the beleaguered Andrew Strauss, taking two wickets in three balls.
Dismissing Chris Rogers, who had looked to be Middlesex's last hope of salvaging something, and then Gareth Berg, the Durham pace bowler appeared to have the home side by the throat at 91 for eight, with a lead of only 50.
It came after a morning in which most of Middlesex's batting had been scarcely more convincing than their returning England captain, whose second-ball dismissal for a duck on Friday was followed by a 25-ball six on Saturday, having been dropped on nought before Onions bowled him for a second time.
"That sort of thing can happen," Finn said, sympathising with his teammate. "People are allowed to bowl good balls and he got the same one twice. But that's cricket and he'll move on to the next game when hopefully he'll get runs."
Onions, driven by his desire for a Test recall two years and three months after injury interrupted his international career, finished with 10 wickets in a match for the first time. But after he had taken a breather after three more overs in the afternoon session, Durham's supporting bowlers really let an opportunity slip away.
With Tim Murtagh swinging bravely and John Simpson, the wicketkeeper, batting cannily in support, the nature of the cricket changed. The edges stopped coming and in 15 overs another 82 were added to the Middlesex total before Murtagh's luck ran out on 45 against what was, in the event, a poor legside ball from Mitch Claydon.
Simpson soon fell too, attempting to lift leg-spinner Scott Borthwick over mid-wicket, but the target for Durham had swelled to 130 when it might easily have been 60.
After Will Smith had fallen to Murtagh in the fifth over of the Durham chase, Paul Collingwood was beaten for pace as Finn began to work up a head of steam and, after Ben Stokes and Michael di Venuto had fallen to catches at the wicket in their attempts to force the pace, it soon became a matter of merely deciding when to shut up shop for the batting side.
Phil Mustard had a go but once Dale Benkenstein and Ian Blackwell had also departed there was no option but to block. It might have been different had Borthwick not been reprieved by a no-ball when he nicked Finn to Strauss at first slip, but then Onions these days is no mug with the bat either.