Whisk eggs: in a bowl, whisk eggs with milk, salt, and pepper until frothy. Set aside.
Cook mushrooms: heat oil in a nonstick skillet over medium heat. Add mushrooms and cook 4 to 5 minutes until water releases and they brown.
Add spices: add garlic powder and onion powder. Cook 1 minute until fragrant.
Wilt spinach: add spinach, stir and cook 1 to 2 minutes until wilted and most moisture evaporates.
Add eggs: pour eggs over mushrooms and spinach. Let sit 20 to 30 seconds.
Fold eggs: gently push eggs from edges to center, folding over cooked bits. Continue until mostly set but still wet. Do not overcook.
Finish: remove from heat. Adjust salt and pepper. Sprinkle with chives. Add cheese if using. Serve warm.