Preheat the griddle on high heat and lightly coat it with avocado oil. Drizzle avocado oil over the T-bone and rub half of the onion soup mix onto both sides of the meat, saving the rest for the vegetables.
Place the steak face side down on the griddle and add slices of butter around it so the butter bastes and crisps the meat. Cook for about 6 minutes, adjusting the time depending on your preferred doneness.
On another section of the griddle set to medium-high, cook the onions and mushrooms with 3 tablespoons of butter, the remaining onion soup mix, and a drizzle of avocado oil. Stir and let them cook until browned and crisp, about 5 minutes.
Flip the steak, add more slices of butter around it, and cook for another 5–6 minutes or until it reaches your desired internal temperature.
Toss the vegetables again, then stand the steak upright on its sides to render the fat, holding each edge for 30 seconds to 1 minute.
Pour half of the beef broth over the vegetables and toss well. Pour the rest of the broth over the steak.
Place the cheese slices on the steak and cover it with a dome until the cheese melts, about 30 seconds to 1 minute.
Transfer the steak to a plate and spoon the vegetables on top.
Sprinkle with crispy onion straws and chopped parsley, then serve immediately with a glass of cabernet or chianti.
