Begin by finely dicing 1/4 cup of onions and thinly slicing 1 1/2 cups of mushrooms. Rinse 1/2 cup fresh baby spinach and set aside.
Heat 1/2 tablespoon olive oil in a nonstick skillet over medium heat. Add the onions and sauté for about 2 minutes until translucent and fragrant.
Toss in the mushrooms and cook until they release their moisture and turn golden brown, about 5 minutes.
Stir in the baby spinach and cook until just wilted, about 1 minute. Remove the vegetable mixture from the skillet and set aside.
In a bowl, crack 2 large eggs and 1 large egg white. Add 1 teaspoon of water, a pinch of kosher salt, and freshly ground black pepper. Whisk vigorously until fully combined and slightly frothy.
Return the skillet to medium-low heat and add a tiny drizzle of olive oil if needed. Pour in the whisked eggs and let them sit undisturbed for about 20 seconds.
Gently stir and fold the eggs with a spatula, bringing cooked edges into the center.
When the eggs are about halfway set, fold in the sautéed mushroom-spinach mixture.
Sprinkle 2 tablespoons of your chosen cheese over the eggs. Continue cooking gently, folding the eggs until they are softly set but still moist and creamy.
Remove from heat immediately to avoid overcooking.
Plate your Mushroom-Spinach Scrambled Eggs right away and enjoy.