Mix the cornstarch with the 2 cheeses in a bowl and set aside. The cornstarch will help prevent the cheese sauce from getting too stringy.
Heat the milk: Put the milk into a small saucepan and heat until steamy. Keep warm while you are preparing the recipe, do not let boil.
Cook the macaroni pasta: Put 2 quarts of water, salted with 1 Tbsp salt, into a large pot and heat to a boil. Once the water is at a full boil, add the macaroni to the pot. Return to a boil, and cook, uncovered, for about 2 minutes less than the macaroni package specifies, so that it is al dente, cooked, but still firm to the bite.
Cook the chorizo and onion: While the water is heating and the macaroni cooking, prepare the chorizo cheese sauce.
Remove chorizo from its casing and put into a medium thick-bottomed saucepan on medium high heat. When the chorizo begins to sizzle, reduce the heat to medium. Cook the chorizo, stirring often, for 2 minutes.
Add the minced red onion and cook the mixture until the onions soften, about 3-4 minutes.
Drain macaroni, preheat oven: When the macaroni is cooked, but still a bit firm, drain it and set aside. Preheat oven to 375°.
Make the chorizo cheese sauce: 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 scraping 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 grated cheese into the chorizo milk sauce.
Bake: Pour the macaroni and cheese into a small casserole dish. Make sure the top is level. Dust the top with the breadcrumbs, then dot with the butter. Bake until the breadcrumbs are golden, about 20 to 25 minutes.
Add the cooked macaroni to the chorizo cheese sauce. Stir in the lime juice.