Char the poblanos: Char the peppers over the open flame of your gas stove, turning occasionally until all sides are blackened. Transfer to a heat proof bowl, cover and let cool to room temperature. If you don't have a gas stove you can also do this on a grill or under the broiler. 

Make the Salsa Ranchera: Turn the broiler to high and line a baking sheet with aluminum foil. Place tomatoes, onion, garlic, and serranos on the baking sheet and place under the broiler. Check every 2-3 minutes and turn the vegetables so the blacken on all sides. The garlic will cook much faster than the other vegetables. Remove each ingredient as it is done cooking. This should take anywhere from 9-15 minutes. 

Blend the salsa ingredients. Remove the skin from the garlic and the stem and skin from the serranos. If you are sensitive to heat you can also remove the seeds of the serrano peppers too. Place all the blackened vegetables into a blender along with the 2 teaspoons of salt, the oregano, and the cup of water. Blend on high until smooth. 

Fry the sauce. Heat the 2 tablespoons of oil in a small Dutch oven or heavy bottomed pot over medium-high heat. Add the sauce (careful it will splatter) and fry in the oil until the color deepens and the sauce is fragrant, about 2-3 minutes. Cover and keep warm. 

Peel the peppers. Carefully remove the blackened skin from the peppers. Try your best not to rip or break them. Get off as much as you can and rinse the rest off under running water. 

Stuff the peppers. Cut a slit down the side of each pepper and remove the seeds with your fingers, again being careful not to rip or break them. Rinse out any remaining seeds under running water. Fill each pepper with 3/4 to 1/2 cup of shredded cheese depending on the size of the pepper—you will use all the cheese. Place 1-2 fresh epazote leaves inside each pepper if using. Seal closed by threading a toothpick through the opening. 

Make the batter. Beat the egg whites in a bowl of a stand mixer with the whisk attachment on medium-high until they are stiff. Turn the mixer to low and add the yolks one at a time until they are completely incorporated. Add a generous pinch of salt and mix that in as well. 

Heat oil. Heat the cup of oil in a large frying pan over medium-high heat. 

Batter the chiles. Place flour in a  pie plate or shallow baking dish. Season generously with salt. Coat the chiles, one at a time, in the flour, dusting off any excess then dip the chiles into the mixing bowl and cover with the egg batter. I like to use a small spatula to "paint" the batter onto the chiles until they are thoroughly coated. 

Fry the chiles. Once the oil is hot (test by sticking a wooden chopstick in the oil, if it is immediately covered in bubbles the oil is ready) add the chiles, two at a time. Adjust the oil temperature as they are cooking. If the oil starts to smoke, turn it down if it becomes too cool, turn up the heat. 

Flip the chiles. Once the chiles are golden brown on one side, flip using a fish spatula and cook until golden on that side. Keep flipping and frying until they are golden all over and they are warmed through, about 5-8 minutes. 

Drain the chiles. Remove to a paper towel-lined baking sheet to drain the excess oil and sprinkle with salt. Repeat with remaining chiles. Then serve immediately with Salsa Ranchera. 

Make in advance. The chiles keep great if you want to make them ahead. Keep them warm for up to an hour in a low (200°F) oven. Or make, let cool, and refrigerate then reheat in a 350°F oven until hot, about 15 minutes.