For the dip: Over medium-low heat, melt the butter in a large saucepan. Add the flour and cook, constantly stirring, for about one minute. Add the chopped onion and Serrano and cook for one more minute. Whisk in the milk slowly and cook, stirring constantly, until thickened, about 3 - 4 minutes. Reduce heat to low. Once the temperature in the pot has gone down some, start adding the shredded Chihuahua and Pepper Jack cheese, cream cheese, salt, spices and crushed red pepper flakes; keep the temprature as low as possible, to prevent the texture from getting grainy (see notes below) and whisk until cheeses have slowly melted and are blended in, about 5 minutes. Stir in chopped poblano peppers.