We have just over 6kgs of sirloin to roast. On the bone.
It looks massive, specially cut to fit the width of the oven, & sitting on the cut face of the bone so that the meat rears out of the pan like the stump of a wall from the ruins.
How long to cook? it’s been two hours out of the larder, so not like putting an ice lolly in the oven. Even though it’s massive, it’s cross section is relatively slight and the meat is divided by a continuous blade bone.
It’s time for O level physics. The key metric is surface area:volume & the higher this ratio the faster the cooking. It is not going to be like doing a steak but nor is it a great lump. In the end we go for 20 mins / kg at 190 deg C before resting for 40 mins. It is wonderfully tender and absolutely delicious, but, sadly, slightly overdone. At some stages in the discussion 3hrs, even 3 hrs 15 were mentioned but that would have been a disaster. On reflection I underestimated the effect of the speedier conduction of heat through bone but not a bad outcome.