Halve green bell pepper lengthwise, and use a spoon to remove seeds and membrane. Halve and peel onion. Slice halves into thin strips. Cut mushrooms into ¼" slices. Mince garlic. Mince parsley, both stems and leaves.
Roast the Peppers
Place peppers on prepared baking sheet, cut side down. Drizzle each pepper with ¼ tsp. olive oil each. Bake until peppers are tender, but not completely cooked, 10-12 minutes. Set aside. Don't worry about a little char on the peppers. It adds flavor to the dish. While peppers roast, cook meat.
Brown the Meat
Heat 1 tsp. olive oil in a large non-stick pan over medium heat. Add ground beef to hot pan, and cook, breaking up meat, until no pink remains, 8-10 minutes. Transfer beef to a mixing bowl. Reserve pan; no need to wipe clean.
Cook the Filling
Heat 1 tsp. olive oil in pan used to brown beef over medium-high heat. Place onions and mushrooms in hot pan and stir occasionally until lightly charred, 4-5 minutes. Add garlic, parsley (reserve a pinch for garnish), and ½ tsp. salt to pan. Stir occasionally until onions are tender, 2-3 minutes. Transfer filling to bowl containing beef. Add mozzarella cheese and ¼ tsp. salt and stir.
Assemble and Bake Peppers
Flip peppers so cut side is up and carefully distribute filling evenly among halves. Top with panko. Drizzle each pepper with ¼ tsp. olive oil and bake until cheese is melted, 8-10 minutes.
Finish the Dish
Place two stuffed pepper halves on a plate. Garnish with remaining parsley.