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Turn on the broiler. Place the poblano peppers on a foil-lined baking sheet and broil for about 10 minutes or until skin is blistered and starting to blacken, turning halfway through. Carefully wrap the peppers tightly in the foil and let them steam for 5 minutes or so. Peel the skin from the peppers and discard the stem and seeds. Chop the peppers and set aside.
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Reduce oven temperature to 425ºF. Spray a square baking dish with cooking spray.
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Melt the butter in a large saucepan over medium heat. Whisk in the flour and cook for 3-5 minutes, or until the mixture is lightly browned. Stir in the garlic and dry mustard, then slowly pour the milk into the saucepan, whisking the entire time.
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Bring the sauce mixture to a slow simmer; let it continue to simmer, stirring frequently, until it's thickened, about 7 minutes or until the sauce coats the back of a wooden spoon.
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Remove the sauce from the heat and whisk in the cheddar cheese. Stir in the chopped peppers and noodles, then season to taste with salt and pepper.
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Pour the mac and cheese into the prepared baking dish. Sprinkle panko over the top and lightly spray with oil. Bake for about 25 minutes, or until the sauce is thick and bubbly and the panko is browned. Sprinkle with cilantro if desired and serve.