Chef John's Stuffed Peppers

INGREDIENTS

• 1 cup uncooked long grain white rice
• 2 cups water
• 1 onion, diced
• 1 tablespoon olive oil
• 2 cups tomato sauce
• 1 cup beef broth
• 1 tablespoon balsamic vinegar
• 1/4 teaspoon crushed red pepper flakes
• 1 pound lean ground beef
• 1/4 pound hot Italian pork sausage, casing removed
• 1 (10 ounce) can diced tomatoes
• 1/4 cup chopped fresh Italian parsley
• 4 cloves garlic, minced
• 2 teaspoons salt
• 1 teaspoon freshly ground black pepper
• 1 pinch ground cayenne pepper
• 4 large green bell pepper, halved and seeded
• 1 cup finely grated Parmigiano-Reggiano cheese