1.Preheat oven to 400 F. Cook chorizo over medium heat in a cast iron pan. Crumble as it cooks, breaking it apart into smaller pieces. Transfer cooked chorizo to a plate lined with paper towels to drain excess fat.
2.Let a thin layer of fat from the chorizo remain in 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.
3.Transfer the mixture into an 8 or 9 inch baking dish. Stir a couple handfuls of cheese into the veggies. Spread the veggies and melted cheese across the bottom of the dish, forming the first "layer". Add a layer of crumbled chorizo, then top with remaining cheese. Reserve a couple of spoonfuls of chorizo.
4.Bake for about five minutes, until cheese has totally melted. Switch the oven to broil for a few minutes to get the bubbly, browned cheesy top layer.
5.Carefully serve the fundido and top with remaining chorizo, and serve with warm tortillas, spoons for scooping, and guacamole, salsa, or pico de gallo if you like!