Whisk together the eggs, milk, salt, and black pepper in a bowl until the mixture is well-combined.
Melt the butter in a medium-sized skillet over medium heat.
Add the sliced mushrooms and sauté until they begin to soften, about 3 minutes.
Toss in the minced green onions and spinach.
Cook until the spinach wilts and the mushrooms are tender.
Remove the vegetables and set them aside.
Pour the egg mixture into the same skillet.
Allow to cook until the eggs start to set at the edges.
This usually takes about 2 to 3 minutes.
Add the sautéed vegetables back into the skillet on top of the eggs.
Sprinkle with your choice of cheese.
Continue cooking until the eggs are fully set and the cheese melts.