Mix the cornstarch with the 2 cheeses in a bowl and set aside. The cornstarch will help prevent the cheese from getting too stringy.
Put the milk into a small saucepan and heat until steamy. Keep warm, do not boil.
Put 2 qts. of water, salted with 1 Tblsp. salt, into a large pot and heat to a boil. Once the water is at a full boil, add the macaroni. Return to a boil, and cook, uncovered for about 2 minutes less than the macaroni package specifies.
While the macaroni is cooking, prepare the chorizo cheese sauce. Remove chorizo from its casing and put in a medium thick-bottomed saucepan on medium high heat. When the chorizo starts to sizzle, reduce heat to medium. Cook, stirring often, for 2 minutes. Add the minced onion and cook the mixture until the onions soften, about 3-4 minutes.
When the macaroni is cooked, but still a little firm, drain it and set aside. Preheat oven to 375*.
Add the flour to the pot with the chorizo and mix well. Everything will clump up. Reduce the heat to medium-low and cook, stirring constantly and scaping the bottom of the pot, for 2 minutes. Add the hot milk a tablespoon at a time, stirring constantly. When all the milk has been added, heat the mixture to a gentle simmer and cook for 2 minutes.
Mix the shredded cheese in to the chorizo milk sauce. Mix in the cooked macaroni into the chorizo cheese sauce. Stir in the lime juice. Pour the mac and cheese into a small casserole dish. Make sure the top is level. Dust the top with breadcrumbs, then dot with the butter. Bake until the breadcrumbs are golden, about 20-25 minutes.