In a large bowl, whisk together the eggs, 1 tablespoon water, and 1/2 teaspoon each salt and pepper.
Heat 1 tablespoon olive oil in a large skillet over medium heat.
Add the onion and 1/4 teaspoon each salt and pepper and cook, covered, stirring occasionally,
Cook until very tender, about 8 minutes.
Increase the heat to medium-high, add the mushrooms and cook, tossing occasionally,
Cook until golden brown and tender, 4 to 5 minutes.
Meanwhile, heat the remaining tablespoon oil in a 10-inch nonstick skillet over medium heat.
Add the eggs and cook, stirring every few seconds with a rubber spatula, to desired doneness
2 to 3 minutes for medium-soft eggs.
Add the spinach to the eggs and cook, folding it in, until beginning to wilt.
Fold in the chopped mushroom and mozzarella.