Heat the oil in a large (10-inch) skillet over medium heat. Add the onion and chorizo and cook, stirring regularly and breaking up any clumps, until the chorizo is cooked through, about 10 to 15 minutes. Add the beer or other liquid and the spinach; stir until the liquid has evaporated and the mixture is once again dry looking.
Reduce the heat to medium-low, sprinkle the cheese evenly over the vegetables and stir slowly and constantly until just melted—too long over the heat and the cheese will become tough, oily and stringy. Immediately scoop into a warm serving dish (a small fondue dish with a tea light below is ideal) and serve with warm tortillas and salsa for making soft tacos.
NOTE: If you use beer, Monterey Jack is the best cheese.