Collingwood claimed 5-57 at Taunton and Onions rattled through the lower order as Somerset failed to make 300 – just – after winning the toss and deciding to bat.
They looked to be in good position with Johann Myburgh scoring a century, but after he fell to Collingwood for 118 and James Hildreth (53) was trapped in front by the former England all-rounder, their innings fell apart.
Collingwood also accounted for debut boy James Allenby (8), ‘keeper Alex Barrow (4) and the dangerous Peter Trego for a duck before Onions added to his early dismissal of Marcus Trescothick by taking the last three wickets to dismiss Somerset for 299 – Onions ended with 4-59.
Lewis Gregory soon had Durham in trouble with two early wickets, but Scott Borthwick and Michael Richardson put on 84 for the third wicket as the visitors reached 98-2.
Luke Wright and Ben Brown starred with the bat for Sussex??as they battled back from early trouble to post 300-6 on the opening day of their clash with Hampshire at the Ageas Bowl.
Wright was dismissed four short of a century and Brown remained unbeaten on 85 after they had been reduced to 22-3.
Gareth Berg took two early wickets on his Hampshire debut after Luke Wells had been bowled by James Tomlinson for a disappointing opening day duck.
But Wright and Brown took up where Ed Joyce (42) and Craig Cachopa (41) left off, with honours fairly even at the end of the first day’s play.
Champions Yorkshire had the better of the day against Worcestershire at New Road, where Jack Brooks and Tim Bresnan took three wickets apiece.
Brooks was in fiery mood early on, reducing the home side to 11-2, but Moeen Ali proved his fitness for England duty and Tom Fell played a starring role with the bat as they made 264-8.
Moeen compiled a fluent 62 after a spell on the sidelines with a abdominal injury, and he could join his international team-mates in the West Indies towards the end of the week.
But it was Fell who secured Worcestershire’s first century of the season, with his 114 coming off 196 balls before he became one of Bresnan’s victims. Gareth Andrew also made a breezy 42 and was unbeaten at stumps.
And Nottinghamshire debutant Brendan Taylor made a fine impression at Lord’s, where he plundered 106 against a tough Middlesex attack.
The former Zimbabwe international held the Notts innings together as James Harris, once an England hopeful, began the season in fine form by taking 4-75.
James Taylor made 36 and Chris Read an impressive 62 not out, but the rest of the Nottinghamshire line-up struggled, with them standing on 298-9 at the close.