Essex 253 for 4 (Westley 134, Chopra 59, Patel 3-41) beat Middlesex 193 (Gubbins 50, Wagner 3-40) by 60 runs
Tom Westley struck his first century of the season to secure a winning start to Essex's Royal London One-Day Cup campaign as they thrashed Middlesex by six wickets at Radlett.
Westley, who has struggled for form in the County Championship, hit 134 and shared a second-wicket stand of 153 with Varun Chopra as last year's semi-finalists cruised home with 7.2 overs to spare.
Middlesex, who tasted defeat at Radlett for only the second time, were left to rue dropping Westley twice - the first of those when he had made only 12.
Stand-in captain Steven Finn's decision to bat after winning the toss looked a sound one, with Jamie Porter sending the first delivery of the innings for four wides and Nick Gubbins quickly getting into his stride.
The in-form Gubbins (50) savaged Porter as he whipped three boundaries through the gully and guided the Middlesex total past 50 in only the eighth over.
He and Stevie Eskinazi looked comfortable in their opening-wicket partnership of 79 - but the picture changed when Eskinazi miscued a hook off Neil Wagner to long-leg.
Wagner induced Paul Stirling to chip to mid-wicket in his next over and, when Simon Harmer found a way through Gubbins' defences, the hosts were on the slide at 86 for 3.
Their hopes of posting a sizeable score rested largely on Eoin Morgan, who swept Dan Lawrence for the only six of the innings and kept the scoreboard ticking over as he added 57 for the fourth wicket with Hilton Cartwright.
But, while of most of Middlesex's batsmen got started, none were able to take the next step - with John Simpson looking well set before he perished to a smart midwicket catch by Chopra.
Harmer trapped Morgan lbw for 45 soon afterwards and, although James Franklin and Tom Helm played sensibly to haul Middlesex to exactly 250, Ravi Bopara polished them off nine balls short of their allotted overs.
Essex's reply got off to an inauspicious start when Adam Wheater was caught behind in Helm's first over - but Westley and Chopra soon got on top of the bowling.
Westley had made just 12 when he flicked Finn to backward square, but Tom Barber put down the catch and the home side paid heavily for that let-off.
With Chopra content to drop anchor, Westley pierced the gaps in the field and motored on to reach his half-century with a crashing cover drive off Cartwright.
He was given his second life on 78 when Stirling failed to hang onto a chance at mid-wicket - but by then the visitors were well on their way to a resounding win.
Chopra eventually departed for 59, stumped off Ravi Patel, who also claimed the late wickets of Westley and Dan Lawrence before Essex sealed victory.