Make the marinade by mixing the broth, garlic powder, Worcestershire, and Tabasco in a bowl with a fitted lid. Cover each steak with the marinade, then close the lid and refrigerate overnight.
Preheat oven to 400 degrees Fahrenheit.
Brown the Cajun trinity of onion, bell pepper, and celery in about 3 tablespoons butter in a 12-inch cast iron skillet. When the trinity is halfway browned, add the sliced mushrooms. When the mushrooms are tender and slightly caramelized, add the minced garlic.
Cook until very fragrant, stirring constantly, for about 2 minutes. Remove from heat.
Remove half of the trinity, garlic, and mushroom mixture. Let cool.
Position one marinated beef top round steak on a clean cutting board. Spread 1 to 2 tablespoons of the Rotel tomatoes over one side of the sliced steak. Top the Rotel tomatoes with spinach leaves, cheese, and the caramelized Cajun trinity, mushroom, and garlic mixture. Roll as tight as you can, then secure with a toothpick.
Repeat the roll up process with the remaining strips of steak. ( I got a total of 8 roll ups, which I think is perfect for 2 hungry people to share.)
Brown each roll up in the same cast iron skillet you used to brown the vegetables.
When all the steak is browned, remove any fat from the pan, but try to keep a small amount of pan drippings in the skillet, if possible.
Add more butter to the pan, make a blond roux by stirring the tapioca starch into the butter, and cooking for about 2 minutes.
Add the remaining browned vegetables and the rest of the Rotel tomatoes to the skillet. (This should immediately have the consistency of a mushroom sauce. If it is too dry, add a tablespoon or two of gluten free beef broth or water. If its too runny, add another teaspoon of tapioca starch to the mixture.)
Top the sauce with the browned roll ups. Bake for about 20 minutes, or until the steak is cooked through and the sauce thickens slightly.