Chef John’s Stuffed Peppers

INGREDIENTS :

1 cup uncooked long grain white rice
2 cups water
Sauce:
1 onion, diced
1 tablespoon olive oil
2 cups marinara sauce
1 cup beef broth
1 tablespoon balsamic vinegar
1/4 teaspoon crushed red pepper flakes
Peppers:
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 peppers, halved lengthwise and seeded
1 cup finely grated Parmigiano-Reggiano cheese, plus more for topping