Place peppers on baking sheet and broil on high 10 minutes, turning occasionally to ensure blistering on all sides. Remove from oven and place peppers in zipped plastic bag 10 minutes to allow skin to loosen. Remove from bag and peel skin from peppers. Remove seeds and chop.
Cook macaroni according to package directions until al dente. Drain and set aside.
In large pan, melt butter over medium-low heat and sprinkle in flour. Cook 2-3 minutes,
stirring constantly. Pour in milk and whisk until smooth. Cook until thick; reduce heat to low.
In small bowl, whisk egg. Add 1/4 cup warm milk and butter mixture slowly and
whisk together to temper egg. Add egg mixture back to pot with the milk and butter and whisk until smooth.
Add shredded cheese to milk and butter mix and stir until melted. Add salt and pepper to taste.
Mix in chopped red peppers and green chilies (do not drain).
Add pasta to cheese sauce and mix to combine. Serve immediately or pour into baking dish, top with additional cheese or crushed tortilla chips and bake at 350°F until cheese is melted, about 20 minutes.