To make the chorizo: In a large sauté pan heat 1 teaspoon of vegetable oil, add in the diced onion, and season with salt and pepper. Cook stirring frequently for 2-3 minutes until the onion softens. Add in Mexican chorizo making sure you break it into chunks as it cooks; stir frequently for 8-10 minutes until the chorizo is crisp. Set aside.
To make the pupusas: In a large bowl combine the masa harina, warm water, and salt. Knead until dough forms. If the mixture is too soft add one tablespoon of masa harina and if the dough is too hard add one tablespoon of warm water until you get a clay-like consistency. Cover the bowl with a wet towel for 10 minutes.
With oiled hands form three-inch balls with the dough. Using your finger make an indent in each ball then fill the dough with 1 tablespoon of Mexican cheese blend and 1 tablespoon of chorizo. Close the indent (if there is any filing leaking cover with a piece of extra dough). Pat the dough back and fourth in your hands until you create a disk like shape. Repeat with the remaining balls of masa harina.
Heat a skillet to medium high heat with 1 teaspoon of olive oil. Cook the pupusas for 2 minutes on each side until golden brown.