I looked around a bit on The Texas Land and Cattle Company. It's a restaurant, correct? I assume that's what you mean here. It's gonna be a little tough, because I've never eaten there, therefore, I canâ€[emoji]8482[/emoji]t even really guess as to its preparation, as I have not seen the dish. I'll give you my best advice, although it sounds like you've done your best already. Perhaps someone who has been there will chime in later. A top sirloin steak is what you need to look for at the grocery store or butcher shop, their site says they use select cut of beef. It is the first cut sirloin, and the tenderest of the sirloin steaks. Sirloin steaks can vary considerably in their cut. The best cuts come from the top portion of the sirloin and are known as top sirloin steak. They are sometimes called top butt steak, hip sirloin, or center cut sirloin. They can be recognized (if purchased whole) by the mid-sized pin bone, which is the long flat part of the hip bone. Generally, the other sirloin configurations with a flat bone are less tender than the top sirloin, but tenderer than the ones with a round bone. Always purchase sirloin steaks with a flat bone if you have a choice. My favorite cut is (what I call) a "baseball" cut of top sirloin (I do beleive any good meat cutter would know what you meant if you asked for it). It is softball sized and shaped, and comes off the very top of the whole sirlion peice. To get a good medium rare, cook the meat to an internal temp of about 125 degrees. After it rests for a few minutes, it should be perfect. If you don't have a thermometer, you can get a cheap one at Walmart, Target, Linens and Things, etc...
As to the cooking method, this is the tricky part. The site says that they grill their steaks over Mesquite. I would then surmise that they smoke with that Mesquite as well. You could call and ask to be sure. The seasonings on the other hand could be a proprietary (secret) blend, or as simple as kosher salt, garlic/onion powder, some type of ground Chile powder and large cracked black pepper. This is only my best guess. If you want to dig, you could call the location, pretend to be a guest who has never been there, and tell them you like the sound of the smoked sirloin but you are allergic to some seasonings. You may get lucky and get an employee who doesnâ€[emoji]8482[/emoji]t know any better and gives you the recipe. Ask what they smoke with.
That's what I would try. Like I said, this one's hard for me because I have never been to the place, or seen the dish. Hey, wait out back one night and pay one of the busboys $10 to "snag" you the recipe..?
If you get anywhere, post your results. The web site looks good.