Trapping Denesh Ramdin lbw made it four wickets in seven deliveries and despite seeing Paul Collingwood drop Lionel Baker at slip, he soon got his man to wrap up the innings and return figures of 5-38 off 9.3 overs.
West Indies were all out for 152, still 225 runs adrift of England's first innings 377. After being forced to follow on, James Anderson reduced the tourists to 39-2 in their second innings by the time bad light ended play for the day.
"Today was a great team effort, we all dug in," said Onions. "Everyone kept patting me on the back and I kept patting other people on the back. It's been a very memorable day, one I'll never forget.
"When I got that ball in my hand and was walking off the field I was going to enjoy it. People kept saying take your hat off and enjoy the moment, and I definitely did that.
England bowlers were exceptional - Dyson
"We had a little toast for me in the changing room, which was very special. It's been a great couple of days - getting selected yesterday and taking five wickets today is just phenomenal."
England will hope to wrap up victory as soon as possible but with rain forecast for Friday, there may yet be hope for West Indies.
The hosts are bidding to win the opening Test of a series for the first time in 14 attempts and Onions is confident they will do just that.
"We've still got a lot of hard work to do," he added. "I got five wickets today and it has been absolutely brilliant but tomorrow we've got to come out and do the right things and if we do I'm sure England will get the win.
"The England dressing room is a great place to be, I'm lucky to be involved at the moment and we're all pretty much a happy family. That's the way it's got to be. It's like that at Durham and maybe it's something I can help bring into the team."
West Indies coach John Dyson admitted England were a very different proposition from the side that had been beaten in the recent Caribbean Test series.
"The start of our first innings was pretty good, and the boys were confident. Then it just came out of nowhere," he said.
"I think the England bowlers bowled exceptionally well today - with pace and intent, and they hit the seam.
"They also caught well, so it was a very impressive display by England. You can see that they are far more comfortable in their own conditions."
"There have been several recent examples of teams appearing down and out at this venue, only to dig in and escape defeat as batting conditions ease.
"It's just got to be an amazing fight - we'll have to bat exceptionally well," he admitted.
"We've just got to hang in, and we might get a little bit of help from the weather."
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