Slice four bell peppers in half, and remove the seeds and stems. Place the peppers in a 9x13” casserole dish, and drizzle them with olive oil. Sprinkle on salt and pepper. Bake for 10 minutes.
While peppers are baking, heat a large skillet on high heat. Pour a thin layer of olive oil in the pan -- just enough to coat the bottom. Generously salt and pepper both sides of the sirloin. Place it in the hot skillet for one minute on each side. Remove from pan and place the steak on a cutting board.
Turn the heat down to medium for the same skillet, and add the mushrooms and garlic. After one minute, add sliced onions and one sliced bell pepper (I used ½ of a red and ½ of a green bell pepper). Season with salt and pepper. Cook for three to four minutes, stirring occasionally.
While mushroom and pepper mixture is cooking, slice the steak into the thin strips, and discard any fat. Add the steak strips to the skillet, and stir to combine.
Turn off the heat to the skillet. Spoon the steak and pepper into each cooked bell pepper half-filling until all halves are full and mixture is used up.
Top with shredded mozzarella cheese, and place in the oven to bake for about 15 minutes. Remove from oven and serve warm.
This recipe is terrific served with cauliflower rice and a green salad.