Heat the oven to 450F with the racks in the lowest and center positions.
Divide the potatoes, red bell pepper, red onion, and rosemary between two rimmed baking sheets. Season each sheet with 1/4 tsp salt, pepper to taste, and 1.5 tbsp olive oil and toss to coat. For maximum crisp, turn the potatoes cut side down. Roast on the lowest and center racks for 30 minutes, swapping the sheets’ position halfway through roasting.
Once the veggies go in the oven, remove the prepared steak from the refrigerator. Allow the steak to sit at room temperature for 10 minutes. (If you did not prepare the steak earlier, prepare it now.)
Heat a 12” stainless steel or cast iron pan over high heat. When the pan is hot, grab the steak with a pair of tongs and press the fat cap down onto the pan to release some of the fat. Sear all areas of the fat cap, then lay the steak onto the pan and apply moderate pressure downward on the meat surrounding the t-bone.
Turn the steak every 30-60 seconds for 5 minutes, keeping the heat high (frequent turning will build a nice brown crust.)
Transfer the pan to the oven’s center rack (remove one of the veggie pans if you need space) and cook 2.5 minutes for medium-rare. Remove from the oven and check the steak’s internal temperature with an instant read or meat thermometer. If the temperature is 120-130F, transfer to a cutting board and tent loosely with foil until the temperature reaches 135F. If the temperature is below 120F, return to the oven for 30-60 seconds more. (See recipe notes in the article above for temperature information for medium and well done steak.)
Once the desired temperature has been reached, allow the steak to rest 10 minutes before slicing. Slice the steak into 1” “ribs” and serve topped with the heart shaped compound butter and flaky salt. Top the roasted vegetables with the watercress and a squeeze of lime juice.