Crack 3 eggs into a bowl. Add a pinch of salt. Whisk vigorously until uniform in color and slightly frothy.
Slice mushrooms. Wilt spinach in a dry pan for 1 minute. Set both aside.
Melt butter in an 8-inch non-stick skillet over medium-low heat until foamy but not browned.
Pour in eggs. Let them sit undisturbed for 30 seconds until the edges begin to set.
Using a silicone spatula, gently push the edges toward the center while tilting the pan to let the uncooked egg flow underneath. Do this around the whole pan.
When eggs are mostly set but still slightly glossy on top, add mushrooms and spinach to one half.
Fold the empty half over the filling and slide onto a plate. The residual heat will finish cooking the interior to a perfect, barely-set texture.