Use tongs to char the green onion over a medium flame, about 1 minute total and remove. Chop the green onion, discarding the ends.
Heat a medium saucepan over medium heat add the charred scallions, beef stock, ketchup, apple cider vinegar, cilantro, garlic and brown sugar and whisk until the brown sugar has dissolved. Simmer until the sauce has thickened, about 20 minutes.
Remove from heat and let cool slightly. Add the sauce to a blender and puree until completely combined. Set sauce aside in a bowl.
For the meatballs:
Preheat the oven to 400ºF (200ºC).
In a small bowl stir together the breadcrumbs and milk until combined and let sit for 2 minutes. In a large mixing bowl combine the beef, onions, chilies, breadcrumb mixture, egg, chili powder, garlic powder, onion powder and salt.
Use your hands to combine the mixture without overworking the meat. Use your hands to roll into large meatballs, each should be about the size of a golf ball.
Place on a baking sheet. Bake the meatballs until cooked through and golden brown, about 20 minutes.
Add the sauce to a large mixing bowl and gently roll the meatballs in the BBQ sauce until they are entirely coated. Set aside.
For the queso:
Heat a medium saucepan over medium heat, add the butter and allow to melt. Add the flour, whisking to combine. Let the roux cook, stirring constantly, until a pale golden brown, about 1 minute.
Add the milk, whisking to combine. Bring the mixture to a simmer, then lower the heat to medium-low, whisking until the sauce thickens, about 5 minutes.
Slowly whisk in the cheddar cheese and once it has all been incorporated stir in the green chilies, tomatoes and cilantro.
For serving:
Add a spoonful of queso to each plate or a large serving platter. Top with meatballs the BBQ meatballs. Spoon additional sauce over the top and garnish with green onions, cilantro, jalapeño and Fritos®.