As with the steak groups other sites, the focus of the menu will be on the eponymous flat iron steak, although the menu will also serve a range of other cuts including sirloin and rib-eye.
Sides will include beef dripping chips; roast aubergine with tomato and basil; creamed spinach; and crispy bone marrow garlic mash
Flat Iron James Street will also serve a new Wagyu beef burger special featuring two patties of grass-fed British Wagyu, topped with melted cheese, pickles, onions and homemade truffle mayo.
Designed by Red Deer, the restaurant will have a 'stripped back' appearance that emphasises the reuse of found objects - Yorkshire sand stone has been repurposed to create the sinks, while the bar will feature ornate glass roofing tiles.
The restaurant will hold 74 covers indoors, with a heated outdoor terrace also available.
"James Street is a really exciting move for us, especially in such a challenging time," says Carroll.
"We have spent these last few months working on some new specials such as the bone marrow mash and Wagyu beef burger.
"Our mindset hasn’t wavered since we started as a pop-up; great steak for everyone, unparalleled quality and doing a small number of things really well. We’re really happy to be bringing this to Marylebone."
Flat Iron opened its first permanent restaurant site in Soho in 2012.
It now operates a total of eight restaurant locations across the capital, seven of which have reopened following the Coronavirus lockdown.