Preheat the broiler to high. Place the poblanos on a broiler pan and broil until blackened on all sides, 12 to 15 minutes. Place in a bowl and cover tightly or place in a paper bag and let cool for 10 minutes. Peel and discard the skin, stems and seeds, then chop the chiles.
Bring a large pot of water to a boil, salt it, add the pasta and cook until al dente. Drain.
While the pasta is working, in a large saucepan, heat the EVOO, 1 turn of the pan, over medium heat. Add the onion and garlic and cook until tender, 6 to 7 minutes. Add the butter to melt, then stir in the flour and cook for 1 minute. Whisk in the milk and chicken stock and bring to a simmer; season with salt and pepper. Add the cumin and coriander and cook until thickened, about 5 minutes. Stir in the cheese with a wooden spoon until it melts.
In a large bowl, toss the pasta with the sauce and roasted poblanos. In a separate bowl, toss the avocado with the lime juice. Serve the mac n' cheese topped with the avocado, tomatoes and scallions.