Preheat grill or oven to 350°.
In a medium pan, add a tablespoon of olive oil and warm pan over medium heat. Add onions and sauté for 2-3 minutes, just until the onions start to become glossy and translucent. Add the sliced jalapeños and continue to sauté, approximately 2-3 minutes more. (*see note)
Add the minced garlic to the pan and stir about a minute, until garlic softens and becomes fragrant. Remove the pan from the heat and set aside.
Generously oil the interior of a 10" cast iron pan with olive oil. Be sure to coat the side of the pan as well as the bottom.
Layer 1/3 of the cubed Monterey Jack cheese along with 1/3 of the cubed Oaxaca cheese into the bottom of the oiled pan. Add a sprinkle of the Spicy Garlapeño Rub. Top with 1/2 of the onion and jalapeño mixture.
Repeat these layers...1/3 of the cubed Monterey Jack and Oaxaca cheeses. Add a sprinkle of Spicy Garlapeño Rub and top with the remaining onion and jalapeño mixture.
Top with the remaining cheese and add a final sprinkle of Spicy Garlapeño Rub.
Place pan on the preheated grill or oven and bake at 350° approximately 20 minutes or until melted through and bubbly.
Serve with tortillas, sliced steak and salsa.