Line a baking pan with aluminum foil. Put the chile poblanos on the lined baking pan and broil for 10 minutes until blackened and charred. Use tongs to flip the chile to the other side. Broil for another 10 minutes. Depending on the thickness of the chile, broil for another 5 minutes on the sides that weren't broiled
Transfer the broiled chile to a brown paper bag and fold to close. Leave in the bag for 5-10 minutes until cooled. You may want to put a plate underneath the paper bag to capture any moisture. In a large skillet, add the oil over medium-high heat.
Carefully peel the blackened skin off the chile as much as you can. Make a slit lengthwise down the middle of the chile (only cut to the tip of the pepper) cutting through the front layer. Remove the seeds to preference. Stuff the chile with the shredded mozzarella cheese and onion.
Separate the egg yolks and egg whites. Whisk the egg whites with a mixer until stiff peaks form. Whisk the egg yolks separately and gradually add the yolks to the whisked egg whites until the mixture is creamy and fluffy. The mixture will be a light-yellow color when ready. Place the flour on a plate and mix with the salt to preference.
Spoon the flour over the stuffed chile. Loosely roll the chile relleno with the flour and slightly squeeze the pepper in your hand for the flour to bind everything together. Dip the chile in the egg mixture and use a large turner to lower the chile into the heated skillet with oil. Fry each side for 1-2 minutes until golden brown. Enjoy!