Preheat the oven to 375°F.
In a large bowl, whisk together the eggs, water, salt, and pepper until well combined. Set aside.
Heat olive oil in a 10-inch oven-safe skillet over medium heat. Add the mushrooms and onion and cook 6-7 minutes, stirring occasionally, until the mushrooms release their liquid and it evaporates.
Add the garlic and cook 30 seconds, then add the spinach in batches, stirring until wilted, about 2 minutes.
Spread the vegetables evenly across the skillet, then pour the egg mixture over the top. Cook undisturbed 2-3 minutes over medium heat until the edges begin to set.
Transfer the skillet to the oven and bake 12-15 minutes, until the eggs are just set in the center and lightly puffed.
Let cool 5 minutes, scatter with chives, then slice into wedges to serve.
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