Preheat broiler to high. Broil the poblanos on a pan until charred on all sides, 12 to 15 minutes. Place in a paper bag and let cool for 10 minutes. Peel, seed, and chop.
Whie the poblanos are under the broiler, bring a large pot of water to a boil, salt the water, add the pasta, and cook according to package directions.
While pasta cooks, heat the oil in a medium saucepan over medium heat. Add the onion and garlic and cook for 6 to 7 minutes, until softened.
Add the butter to the saucepan; stir to melt. Whisk in the flour and cook for 1 minute. Whisk in the chicken broth, then the milk, and bring to a simmer. Season with the cumin, coriander, salt, and pepper. Cook until thickened, about 5 minutes. Add the cheese and melt it into the other ingredients.
Toss the pasta with the sauce and the reserved poblanos. Serve in shallow bowls topped with tomatoes, avocado, scallions, and the lime juice.