McWin and TriSpan have teamed up to take a co-controlling majority stake in Flat Iron restaurants.
The transaction will see Piper, which invested £10m in Flat Iron in 2017, leave the company.
McWin and TriSpan plan to expand Flat Iron across the UK and take the brand international for the first time.
Flat Iron, launched by Charlie Carroll in 2012, now operates 18 locations in London, Manchester, Leeds, and Cambridge. Its signature £10 steaks have since risen to £15.
Tom Byng, leading Flat Iron since 2021, will stay on as CEO as the brand embarks on its next growth phase.
Byng said: “Our success at Flat Iron is a team effort from the pot wash to the boardroom, and we are pleased to welcome McWin and TriSpan to the Flat Iron family.
“Their deep experience and knowledge of the sector are second to none and will be invaluable as we continue our mission to serve ‘remarkable steak’ to more locations in London, across the UK and abroad.”
McWin also holds investments in other restaurant brands, including Gail’s, Sticks’n’Sushi, and Big Mamma Group.
Flat Iron becomes the first buy for McWin’s Foodservice Fund, which focuses on scaling lower- to mid-cap European restaurant brands.
TriSpan made the investment via its Rising Stars Programme, which has previously supported restaurant brands including Pho, Mowgli, and Rosa’s Thai.
Robin Rowland, operating partner at TriSpan, said: “The UK dining out market is a vibrant growing sector of £110b+ turnover that continues to innovate, and Flat Iron is a clear winning brand offering outstanding value that truly delivers to a wide group of guests throughout the week.”
Harry Goss, partner and head of foodservice at McWin, added: “We believe that Flat Iron has the right to become a multi-national British champion.”
Flat Iron was advised on the deal by Houlihan Lokey.
The terms of the transaction were not disclosed.