Flat brisket is the leaner, even slab of the brisket — it slices neatly for sandwiches once the collagen has melted. It is not a hot-and-fast steak. Flat Brisket 1.5kg is a 1.5kg flat; Brisket Joint 1kg is the smaller joint when you want less. Both sit in brisket and beef joints.
Oven low and slow
Salt ahead when you can. Roast fat-side up at 150°C, loosely foiled, for 3.5 to 4.5 hours until a probe slides in with little resistance — often around 90–95°C for slicing tenderness. Rest in the foil for 30 minutes. For a firmer bark, uncover for the last 45 minutes and finish near 160°C. Slice across the grain in pencil-thick slices; with the grain gives you stringy chew even when the cook was right.
Slow cooker and BBQ
Slow cooker: low 8 to 10 hours with onion and stock, then a short hot oven if you want a crust. BBQ: indirect around 120°C until tender; wrap when the bark is set if the cook stalls near 70°C internal. A hard pan sear alone will not finish a whole flat — that is for cubes in a stew. Patience is the method.
What to serve
Pickles, mustard, soft potatoes, or piled into a roll. Leftover cold brisket reheats gently in its juices. Browse slow cooker meat for other long beef cooks, and compare with topside on beef joints when the budget is tighter.
Times above are a guide for home ovens and fryers — always trust a probe over the clock, and rest meat before you carve so the juices stay in the slice, not on the board. British packs vary a little in thickness, so start checking a few minutes early the first time you cook a new size, then write your own note on the fridge.