For the Salsa:
In a large pot, fill it with water and add the guajillo chilies first; boil for 10 minutes. Then add in all the other salsa ingredients (roma tomatoes, white onion, garlic, serranos or jalapeños) minus the salt and cilantro. Let it come to a boil and boil until the tomatoes are softened, about 12-16 minutes. Transfer all ingredients to a blender, then add in 1/2 bunch cilantro (stems included) and a big pinch of salt. Blend on high for 1 minute. Taste and add more salt to your liking.
For the Chiles:
1. You could either roast your peppers on an open flame, rotating them to get evenly charred, OR preheat the oven to 500 degrees F (260 degrees C). Line a baking sheet with aluminum foil and add the poblano peppers. Roast poblano peppers in the preheated oven for 20 minutes; flip and continue roasting until the skin is charred and the flesh is soft, about 20 minutes more.
2. Place roasted peppers in a bowl, cover the bowl with plastic wrap; let them steam for 10 minutes. Peel the blackened skins from the peppers with a damp paper towel, cut a slit on one side, and carefully remove the seeds, keeping the shape of the pepper whole and intact. Stuff each chili with a generous amount of cheese and close up the pepper. (Optional: use toothpicks to seal, though the batter can help).
For the Batter:
Combine 1 cup flour, 1 cup sparkling water, 2 tbsp corn starch, 1 tsp baking powder, 1 tbsp paprika, 1 tsp garlic powder, 1 tsp onion powder, 1 tsp salt, and 1/2 tsp ground pepper in a bowl and whisk until smooth.
Frying the Rellenos (Inferred final step, as it was stated to be in comments):
Heat oil for frying in a deep pot or skillet over medium-high heat. Dip each stuffed poblano pepper into the prepared batter, ensuring it's fully coated. Carefully place the battered pepper into the hot oil and fry until golden brown and crispy on all sides, turning as needed. This usually takes 3-5 minutes per pepper. Remove and drain on a wire rack or paper towels. Serve hot with the prepared Salsa Roja.
