Preheat the oven to 180°C (Gas Mark 4 / 350°F).
Melt the butter in a frying pan, and add the chopped mushrooms, onion and pepper. Cook for around 5 minutes, until fairly soft.
Divide all of the vegetable mixture and most of the cheese evenly across the six tortillas, spooning it along the centre of each. Roll each tortilla tightly, and place seam side down in a lightly greased baking dish.
Lightly beat the eggs with the milk, chilli powder and garlic powder. Pour the mixture over the tortillas, making sure it gets inside each rolled tortilla. Sprinkle any remaining cheese on top, and add a pinch of black pepper.
Bake for around 40 minutes, or until the eggs are completely set and the cheese is golden brown. Leave to stand for at least 5 minutes before serving.