Preheat oven to 350 degrees F (175 degrees C).
Heat a large skillet over medium-high heat. Cook and stir beef in the hot skillet until browned and crumbly, 5 to 7 minutes; drain and discard grease. Add water and taco seasoning to ground beef; cook and stir until until water evaporates, about 5 minutes.
Mix ground beef mixture, rice, tomato soup, tomato, mushrooms, red onion, feta cheese, and green onions together in a bowl; spoon into green bell peppers. Arrange stuffed peppers in a baking dish.
Bake in the preheated oven until peppers are softened and filling is cooked through, about 1 hour. Serve with kalamata olives on the side.