Scoop the mushrooms into a small bowl, cover with boiling water, weight with a plate to keep the mushrooms submerged and let rehydrate for 20 minutes. Drain off the liquid, pressing on the mushrooms to remove all the water. Chop into 1/4-inch pieces.
Finely chop the chiles (seed them first if you wish), then chop the onion and tomato into 1/4-inch pieces. Heat the oil in a medium skillet over medium-high heat. Add the chiles, onion, tomato and mushrooms and cook, stirring nearly constantly, until the onion begins to soften and brown, 7 or 8 minutes. Add the beer and 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 for making soft tacos.