Bring 2 1/2 cups water to a boil in a saucepan; add rice and stir. Reduce heat, cover, and simmer until the rice is tender, 20 minutes.

Preheat oven to 350°F (175°C). Drizzle olive oil in a 9x13-inch casserole dish.

Fill a separate saucepan with water and bring to a boil; add green bell peppers and simmer for 3 minutes. Transfer peppers to a bowl of ice water until cooled. Arrange peppers in the prepared casserole dish.

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. Return skillet to heat; mix onion and cooked rice into ground beef. Pour tomato sauce into ground beef mixture and stir to combine.

Mix garlic, coriander, cumin, cayenne pepper, salt, and black pepper into ground beef mixture. Simmer until flavors have combined, 10 minutes. Remove from heat and spoon 2 to 3 tablespoons ground beef mixture into each bell pepper half. Sprinkle mozzarella cheese over each filled pepper. Cover casserole dish with aluminum foil.