Cut tops off bell peppers (about 1/2-inch down from top). Chop pepper tops discarding stems. Discard seeds from pepper bottoms.
Place peppers in a shallow microwave-safe baking dish. Microwave uncovered 4 to 5 minutes or until crisp tender; set aside to cool.
Heat oven to 375°F. Meanwhile, heat oil in a large saucepan over medium heat. Add chopped pepper tops; sauté 2 minutes. Add rice; sauté 1 minute. Add broth; bring to a boil over high heat. Reduce heat; cover and simmer 15 minutes. Stir pepperoni into rice. Cover; simmer 2 minutes or until rice is tender and liquid is absorbed. Stir in spinach until wilted. Stir in 1/2 cup of the cheese. Drain peppers; stand upright in same baking dish.
Spoon rice mixture into peppers; top with remaining 1/2 cup cheese. Bake 20 to 25 minutes or until heated through.