Crack the eggs into a bowl, add the milk, and beat them well. Melt the butter in a 7-inch nonstick skillet over medium-high heat. Add the eggs and scramble slightly. Allow the eggs to coat the pan and cook evenly. Crumble the bacon into the eggs, add the mushrooms and cheese, and cook for 1 minute. Carefully slide 1 end of the omelet onto a plate while folding the other end over on top, like a taco. Garnish with scallions and serve immediately.