On a hot grill, under a broiler, or over an open flame on the stove top, char the outside of the peppers completely. Transfer to a brown paper bag or zip top plastic bag. Close bag to seal and let peppers rest for 10 minutes. When peppers are cool enough to handle, use a small paring knife to gently scrape off and discard the outer skins. Set 2 roasted peppers aside for sauce.
Make a small incision down one side of remaining 6 chiles and open partially. Carefully remove any membrane threads and seeds. Set each aside on parchment paper or silicone mat-lined baking sheet.
Preheat oven to 250°F.
Add vegetable oil to sauté pan over medium heat. When oil is hot, add the onion and saute until golden. Add garlic, chopped apple and tomato paste, and cook for 3 minutes. Add ground chicken, chile powder, salt and cumin, and continue cooking for about 5 minutes, breaking meat up into crumbles.
Sprinkle flour over chicken mixture and stir to incorporate. Slowly whisk in chicken stock, then simmer mixture until sauce has thickened, about 5 minutes. Remove pan from heat and stir in pepitas. Set aside to cool slightly.
Stuff each poblano pepper with chicken mixture, then gently bring the sides together so pepper encases filling. Place sheet pan of chicken stuffed poblano peppers into oven to keep warm while you make the sauce.
To a high speed blender or food processor, add remaining 1/2 onion and 2 reserved poblano chiles, then puree. Add heavy cream and pulse to blend for a few seconds. Pour cream sauce into a saute pan, bring to a boil, then reduce heat to simmer and cook until reduced and thickened. Season with salt and black pepper to taste.
Remove stuffed peppers from oven and transfer to serving plates. Pour some green chile cream sauce on top of each stuffed poblano pepper and garnish with chopped cilantro and crumbled cotija cheese or queso fresco.