1. Working one at a time, cut off top 1/2 inch of bell peppers. Discard stem and seeds. Chop pepper tops into 1/4-inch pieces; reserve pepper cups. Microwave broth and rice in covered large bowl until liquid is absorbed and rice is nearly tender, 13 to 15 minutes.
2. Meanwhile, cook sausage in 12-inch nonstick skillet over medium-high heat, breaking up meat into small pieces, until browned, 6 to 8 minutes. Using slotted spoon, transfer sausage to paper towel-lined plate.3. Pour off all but 1 tablespoon fat from skillet. Add onions and chopped pepper and cook until browned, 8 to 10 minutes. Stir in garlic, oregano, 1/2 teaspoon salt, 1/4 teaspoon pepper, and pepper flakes and cook until fragrant, about 30 seconds. Add tomatoes, bring to boil and remove from heat.
4. Combine 1 cup sauce, sausage and 1 cup Parmesan with par-cooked rice. Transfer remaining sauce to slow cooker. Using skewer, poke 4 holes in bottom of each reserved pepper cup. Fill each pepper with one-quarter of rice mixture and place upright in slow cooker. Top pepper with remaining 1/4 cup Parmesan. Cover and cook on low until peppers and rice are tender, 4 to 4-1/2 hours.
5. Transfer peppers to plate. Stir basil into sauce and season with salt and pepper to taste. Spoon sauce over peppers. Serve.