Preheat oven to 400 F.
Cook chorizo over medium heat in a small cast iron pan (an 8 or 9 inch pan). Crumble as it cooks, breaking it apart into smaller pieces. Transfer cooked chorizo to a plate lined with paper towels to drain excess fats.
Let a thin layer of fat from the chorizo remain the the pan. Add onion, jalapeño, and a pinch of salt. Cook, stirring occasionally, until softened and fragrant - about 5-7 minutes. Add garlic, and cook for another minute.
Remove the pan from the heat. Stir a couple handfuls of cheese into the veggies. Spread the veggies and melted cheese across the bottom of the pan, forming the first "layer". Add a layer of crumbled chorizo, then top with remaining cheese. (Reserve a couple of spoonfuls of chorizo to top the fundido when it's done.)
Bake at 400 F for about five minutes, until cheese has totally melted. Switch the oven to broil for a few minutes if you want the bubbly, browned cheesy top layer.
Carefully serve the fundido in the cast iron skillet to keep it warm. Top with remaining chorizo, and serve with warm tortillas, spoons for scooping, and guacamole, salsa, or pico de gallo if you like!