Gather all ingredients.

Roast whole peppers over an open flame on a gas stovetop burner, turning frequently, until skins are evenly black and blistered. Transfer peppers to a resealable plastic bag. Seal the bag and let peppers sweat for a while; this will allow the skins to peel off easily.

While the peppers are sweating, cook ground beef in a skillet over medium-high heat until evenly browned and crumbly. Add onion and garlic, and cook for a few more minutes. Season with salt and pepper.

Wear food-safe gloves if desired and remove peppers from the bag. Peel off and discard burnt skins. Run peppers under cool running water to rinse away any burnt pieces, then blot dry with a paper towel. Make a small vertical slit in the side of each pepper. Remove the seeds and veins through the slits.

Stuff each pepper halfway with beef mixture, then fill the rest of the way with shredded mozzarella. Close the slits and secure them with toothpicks.

Beat egg whites in a large glass or metal bowl until thick and fluffy. Beat in egg yolks until blended in, about 1 minute. Place flour in a shallow bowl.