Preheat the oven to 350 ℉. Place the cut-in-half bell peppers in a single layer in a casserole dish. Add a splash of water to the bottom of the casserole dish, cover, and bake for 20 minutes or until bell peppers are soft.
Meanwhile, prep the remaining ingredients. Place the olive oil in a skillet or griddle over medium-high heat. Add the sliced bell peppers, onions, and mushrooms with a big pinch of salt and sautee uncovered until onions are very soft (about 5-7 minutes). Stirring occasionally.
Salt both sides of the sirloin before slicing. Melt butter in a skillet or griddle over medium-high heat. Add the sliced sirloin, working in batches if necessary, and sprinkle half of the okie rub on top. Cook for 2-3 minutes, flip, sprinkle remaining okie rub, and cook for 2-3 minutes.
Carefully pour out any remaining water from the casserole pan after removing the bell peppers from the oven. Using tongs, divide the mushroom mixture among the bell peppers. Then divide the sirloin evenly. Top with provolone shreds or slices.
Bake uncovered for about 3-5 minutes, or until cheese melts. Remove from the oven, top with green onion, serve immediately and enjoy!