Dice the onion and green bell pepper, slice the mushrooms if needed, and preheat your Blackstone griddle to low heat.
Add the bacon fat to the griddle, then cook the onion, bell pepper, and mushrooms for 3–4 minutes, stirring occasionally, until the mushrooms release moisture; lightly season the vegetables and move them to a cooler side of the griddle.
In a bowl, whisk together the beef broth, Worcestershire sauce, and cornstarch, adding beef bouillon powder if using, and set the sauce aside.
Spread butter on the buns and toast them on the griddle until golden brown, then remove and set aside.
Increase the griddle heat to medium-high, add the avocado oil, and spread the shaved ribeye in an even layer; season with steak seasoning, sear for 1–2 minutes, then flip and chop with spatulas until fully cooked.
Add the sautéed vegetables back to the cooked beef, mix well, pour the prepared sauce over the mixture, and stir until it thickens into a rich gravy.
Lay the provolone and white American cheese over the beef and vegetable mixture, allow it to melt, and fold it into the mixture until everything is combined and creamy.
Spread mayonnaise on the toasted buns, load them with the cheesy beef mixture, and top with pickled jalapeños if desired.
