1. Place the poblano peppers on a large baking sheet lined with aluminum foil. For proper roasting, set your oven rack directly beneath the broiler and turn the broiler on.
2. Broil poblanos for 5 minutes, or until the skin is blackened and blistered. Carefully flip them over and broil for another 5 minutes, or until the skin is blackened and blistered.
3. Remove them from the oven and loosely cover the baking sheet with aluminum foil or plastic wrap to keep in some of the heat and help them steam. Let them sit for 5 minutes.
4. Peel and rub off as much of the loose skin on the peppers as possible. It doesn’t have to be perfect, just as much as you can.
5. Cut a small slit down the middle of the peppers with a knife. If you don’t want the peppers too spicy, you can remove some of the seeds; however, poblanos aren’t typically super spicy. 
6. Carefully stuff the peppers with the cheese. Some of the peppers will be extra fragile because of the roasting process and may tear, but that’s okay. 
7. Gently close the opening of the peppers and secure it shut with toothpicks. Use as many as you need to ensure that the cheese won’t ooze out during the frying process. Set aside.
8. Heat the frying oil in a large saucepan or deep skillet over medium-high heat. Line a baking sheet with paper towels and set aside. While oil is heating up, prepare the batter.
9. To make the batter, separate the egg whites from the yolks into two separate bowls — the whites in a large mixing bowl and the yolks in a small bowl. Using an electric hand mixer, beat the egg whites until stiff peaks form.
10. While continuing to beat the egg whites on low, add in the egg yolks one at a time until all of them are fully mixed in. The batter should be light, fluffy and smooth.
11. To coat the peppers, mix the flour and salt in a separate shallow bowl or plate. Gently roll the stuffed peppers in the flour mixture, then give them a gentle tap to remove any excess. Then dip them in the egg batter and gently place them in the hot frying oil.
12. Fry the peppers for 3-5 minutes per side, until the batter is golden brown and crispy. Try not to overcrowd the peppers in the pan — one or two at a time is best, depending on the size of the pan. 
13. Transfer the peppers to the lined baking sheet to drain off any excess oil.
14. To make the salsa roja, mix the tomatoes, onion, garlic, jalapeno, cilantro and kosher salt in a blender. Puree until smooth. Heat the olive oil in a small saucepan over medium-high heat. Add blended salsa and bring to a boil. Reduce heat to low and simmer, stirring occasionally for 8 minutes. Remove from heat, taste and season with more salt if desired.
15. Serve the chile rellenos immediately, on a plate with the salsa roja, garnished with freshly chopped cilantro and some cotija cheese. It’s easiest to remove the toothpicks when you first cut into the peppers, or remove them just before serving onto plates.