How to Make It
In a medium saucepan over moderate heat, combine the butter and onion and cook, stirring occasionally, until softened, 6 to 8 minutes.
Stir in the flour and cook, stirring constantly, for 1 minute. Add the cream and cook, stirring, until slightly thickened, about 5 minutes. Add the cheese, and stir until incorporated. Season the sauce with salt and pepper, and keep it warm over very low heat.
Meanwhile, in a medium pot of boiling well-salted water, cook the pasta until al dente. Drain well and stir into the sauce, along with the green chiles. Serve immediately.