Add the ground beef, chopped mushrooms, steak seasoning, and freshly ground black pepper to a large skillet and cook over medium high heat. Stir with a wooden spoon to break up the meat until the meat is cooked through, about 8 minutes. Drain off any excess fat. Transfer to either a 13x9 or deep dish 9x9 casserole baking dish. Taste to see if it needs any more salt.
Prepare the bread topping by whisking the eggs, coconut milk and olive oil together in a medium size bowl (or 2-cup measuring glass). Add the flours, guar gum and salt to the liquid mixture and stir until well combined.
Next assemble the casserole. Add the onion and pepper mixture over the meat in the casserole dish. Add the cheese slices on top of the veggies, layering the cheese over each other slices if necessary. Pour the bread topping next. Place the casserole dish in the oven and bake for about 30 minutes.
Let sit for a few minutes after pulling it from the oven before serving. Enjoy!