Start by dicing the Roma tomatoes and the onion. Add the tomatoes and onion to a small sauce pan and simmer on low.
You will need to remove the seeds from the peppers. You can do this before you roast them or after, it's up to you. To remove the seeds simply cut a "T" shape near the top of the pepper and cut out the ball of seeds at the top of the pepper.
If you have a gas stove, roast the poblano peppers on an open flame as seen in the photos. You can do this on an outdoor grill or you can also roast them inside your oven in a roasted pan too. Flip them as needed. You will see the outside skin become charred and blackened. This takes about 5 to 7 minutes.
Once you have turned them completely around to cook on all sides, place them in a bowl and secure the bowl with a top (use a plate or a cookie sheet). This will allow the peppers to steam and soften.
Once the peppers have cooled down, remove them from the bowl and use a knife to scrap off the charred blackened skin of the pepper. Now you will stuff the peppers with cheese. I load up and make sure it's completely filled. Set the peppers aside.
You will need to start to whip the eggs for the batter but this is where I begin to heat the oil so as soon as my batter is ready, my oil should be hot and ready to go.
Next, whip the egg whites only on high until you get a stiff peak as seen in the photo. Once you have the peak in the egg whites, add the yolks to the white and fold them in to create the batter for the peppers.
Heat the oil in a deep dish frying pan until it's hot. When it's ready, dip the peppers into the batter and begin to fry them. If your pan is big enough you could do 2 peppers at a time but it's recommended to fry one at a time. Use the spatula to coat the better if it doesn't stick to the pepper. Basically it's almost like piling batter on top of the pepper.
Fry it until it's a golden brown (about 2 to 3 minutes).
When they are ready transfer them to a rack to allow the excess oil to drip off. If you are making many, you can place them in the oven on 250 degrees to keep them warm while you cook the rest of them.
By the time you are done cooking all of your Chiles Rellenos, the sauce should be near done. Add two teaspoons of fresh garlic right before it's done. You can add this mixture to the blender and create a smooth sauce or you can choose to serve it chunky as it is.
Serve each pepper with a serving of sauce and enjoy immediately.