Rub the Poblano pepper with 1 teaspoon olive oil and char over a flame on your stove until completely blackened. Place in a plastic bag for five minutes. Remove from bag and peel off the charred skin. Cut the pepper into thin strips and set aside.
Shred the Oaxaca cheese into small strips.
Heat 1 tablespoon of olive oil in a small sauté pan; add the sliced red onion and cook until translucent about 3 minutes.
Thinly slice the Spanish chorizo lengthwise. Lightly butter one side of the two sour dough bread slices.
Heat a frying pan with medium-low heat.
Place a slice of the bread with the butter down in the frying pan. Assemble the sandwich layering all of the ingredients. Top with the other slice of bread with the buttered side up.
Heat sandwich until the bread is golden brown and the cheese starts to melt. Be careful turning the sandwich over as you do not want all of the ingredients to come out.
Cut in half and serve.