Put the poblano peppers on a baking sheet lined with aluminum foil. Place the baking sheet on the highest rack in the oven and broil on high for 15 minutes, flipping every 5 minutes.
Once the peppers have roasted, remove them from the oven and place them in a sealable plastic bag for 10 minutes to steam.
Turn on the oven to 375°F to continue warming.
While the peppers are roasting, add all of the sauce ingredients to a saucepan and slowly bring to a boil.
Add the sauce to a food processor and pulse about 10 times to smooth out the sauce but still leave a few chunks of vegetables.
Place about 2/3 of the sauce at the bottom of a cast-iron skillet or oven-safe pan/dish.
Remove the peppers from the plastic bag, cut a slit in the side, and remove the seeds and top stem. Flip the pepper over and peel off as much charred skin as you can. Be careful, the peppers are still hot!
Flip the pepper back over and fill it with a generous amount of cheese. I used a mix of queso fresco, sharp cheddar, and mozzarella.
Roll up the sides of the pepper and place it open side down in the pan with the sauce. Don’t worry if it breaks a little, it will be covered with more sauce and cheese.
Cover the cheese-stuffed peppers in the pan with the rest of the sauce and cheese.
Bake the rellenos in the oven at 375°F for 25 minutes. Remove the pan from the oven and serve with chopped green onion, avocado, and sour cream. Enjoy!