Bring a large pot of water to a boil.

Cook the green bell peppers in boiling water until slightly softened, about 4 to 5 minutes.

Remove peppers from boiling water and rinse with cold water to stop cooking; set aside to drain.

In a large bowl, combine the lean ground beef, cooked white rice, lightly beaten egg, finely chopped onion, Italian-style bread crumbs, salt, black pepper, garlic powder, and 1/4 cup grated Parmesan cheese.

Mix until all ingredients are evenly distributed.

Preheat oven to 350 degrees F (175 degrees C).

Divide the beef and rice mixture evenly among the prepared bell peppers.

Stuff each pepper generously with the mixture.

Place the stuffed peppers open side up in a 9x13-inch baking dish or shallow roasting pan.

In a separate bowl, combine the tomato sauce and white vinegar.

Pour the tomato sauce mixture evenly over the stuffed peppers.

Sprinkle the remaining 2 tablespoons of grated Parmesan cheese over the tops of the peppers.

Bake in the preheated oven until the peppers are heated through and tender, basting with the sauce two or three times during baking, approximately 80 minutes.