Slice the mushrooms evenly and rinse the spinach well. Crack the eggs into a bowl and whisk until fully combined and slightly frothy.
Heat a nonstick skillet over medium heat and add the butter or olive oil.
Add the mushrooms to the pan and cook for 2–3 minutes, stirring occasionally, until they release their moisture and turn lightly golden.
Add the spinach and cook for about 30 seconds, stirring just until wilted.
Reduce the heat to low and pour the eggs into the skillet.
Gently stir with a spatula, scraping the bottom of the pan as soft curds form.
Season with salt and black pepper near the end of cooking.
Remove from heat while the eggs are still slightly soft, allowing residual heat to finish cooking them.