To start with, choose the reverse sear method: first pre-cook with moderate, indirect heat, then finish grilling at high temperature directly over the coals. Keep the temperature in the BBQ at around 120 degrees during indirect cooking. For a subtle smoke aroma, put a small block of smoking wood on the coals. Oak or beech are both fine. Do not overdo it: smoke should support, not dominate.
Do not let the meat come to room temperature first. The T-bone goes cold from the fridge onto the grill. A cold surface absorbs smoke better and also heats up the meat in a more controlled way during the indirect phase. Rub the meat firmly with some coarse sea salt.
And then the trick: put something insulating under the part of tenderloin. Aluminium foil does not work, as it actually conducts heat. You can use a thin slice of potato or celeriac, for example. Due to the high moisture content and low thermal conductivity, this acts as a thermal buffer. This way, the tenderloin cooks less quickly than the entrecote, which is exactly what you want to achieve.
Let the T-bone cook gently indirectly and measure both sides separately. When the core temperature of the tenderloin has reached 45 degrees, the core temperature in the tenderloin should be a few degrees lower. Actually check that; if the difference is not there, adjust the insulation slightly.
Convert the BBQ for direct grilling. Let the temperature rise to about 250 degrees and, if you have one at least, place a cast-iron grate to grill the T-bone off. Do not close the lid or hood of the BBQ again when grilling off. All you want to do is sear the meat and let the core temperature rise a few degrees in a controlled manner. In a closed BBQ, this goes way too fast and the meat cooks too far.
For a nice grill, turn the meat a quarter turn after half a minute. Another half a minute later you turn the meat over and half a minute later you turn it another quarter turn. The grill window does little for the flavour, but it does look very professional. Moreover, such a grill window is much easier to create on a cast-iron grid than on stainless steel. Cast iron simply conducts heat better.
Remove the T-bone from the BBQ as soon as the core temperature of the tenderloin has risen to 50 degrees. If all goes well, the core temperature of the sirloin should already be a few degrees higher, towards 54-55 degrees. Then let the meat rest for another 3-5 minutes under a loose piece of foil. While it is resting, the core temperature will rise a little more due to post-cooking; you have calculated that exactly.
I always cut the T-bone meat loose from the bone on both sides before serving. Then I make nice slices of it, about a centimetre thick. Perpendicular to the thread of the meat, of course, always. That thread runs parallel to the leg of the T-bone. Put the sliced meat back around the bone. Thanks to the reverse sear method, you have a beautiful crust while the cooking inside is nice and even.
In the perfect T-bone steak, both tenderloin and sirloin are at just the right core temperature. This is one way to achieve that - without guessing, but with measuring and a bit of clever thinking. And since such a T-bone is big enough for two diners, you get to fight it out with each other over who gets the most of which part. My advice: share fairly. They are both equally delicious.
