Place chile directly over flame of a gas burner (no pan), or on a rimmed baking sheet under the broiler. Cook, turning occasionally until completely blackened, about 4 minutes on the burner or 8 minutes under the broiler. Place roasted chile in a bowl and cover with a plate. Let rest for 5 minutes. Peel under cool running water and discard stem and seeds. Chop finely and set aside.
Toss cheeses with cornstarch in a large bowl until evenly coated.
Heat oil in a medium saucepan over medium-high heat until shimmering. Add ground chuck and cook, stirring occasionally until no longer pink, about 5 minutes. Add cumin and pepper flakes and cook, stirring constantly until fragrant, about 30 seconds.
Add tomatoes, poblano, and milk and stir to combine. Bring mixture to a boil, reduce heat to low, and add cheese. Cook, stirring constantly with a wooden spoon until cheese is completely melted and sauce is smooth. Stir in Frank's, scallions, and cilantro. Serve immediately with crackers, vegetables, and tortilla chips.
