Preheat the oven to 350 degrees F. Lightly grease a square baking dish or cover it with foil. Slice off the tops of the bell peppers and remove the seeds. Place the peppers in the baking dish so that they sit upright. If a pepper is not remaining upright, use a knife to remove a very thin slice from the uneven area of the bottom so it can lay flat. Be careful not to cut a hole or else the eggs will leak through.
In a large skillet, heat a tablespoon of olive oil over medium heat. Add the cubed potatoes and cook for about 15 minutes, flipping halfway through, until browned and tender. Add more olive oil as needed. Stir in the chopped mushrooms, spinach, and garlic, and cook until the spinach has cooked down. Season with salt and pepper as desired. Remove from heat. While the veggies are cooking, whisk together the eggs, salt, pepper, and parsley in a measuring cup (for easy pouring).
Spoon the filling evenly into each pepper. Pour the egg mixture into each pepper, being careful not to overflow them. Sprinkle the cheese on top of the peppers. Bake in the oven for 40 to 45 minutes until the eggs are set.
Remove them from the oven, let cool for a few minutes, and enjoy!