Cook pasta according to package directions. Meanwhile, broil poblano chile 8 minutes or until skin is blistered and blackening; turning twice.
Remove pepper from oven and wrap in clean dish towel 5 minutes to steam. Remove stem and seeds of pepper and chop; set aside.
Heat oven to 350°F. In medium saucepan melt butter. Add onion and cook 2 minutes.
Whisk in flour and dry mustard; cook 1 minute and whisk in milk. Stir in Monterey Jack cheese and 2 cups of the cheddar cheese until melted.
When melted, cook and stir 3 minutes or until sauce is thickened. Add cheese sauce to cooked pasta.
Transfer mixture to sprayed 9×13 glass baking dish. Sprinkle with remaining 1 cup cheddar cheese and tortilla chips.
Bake 15 minutes or until cheese is melted and chips are golden brown. Garnish with cilantro, if desired.