Break all 4 eggs into a bowl and beat with a fork or whisk until well combined. Add a splash of milk to the eggs and whisk together.
Heat 1/2 tbs of butter in an 8″ omelette pan over medium-high heat until butter is sizzling. Add spinach and mushrooms and sauté for 2 minutes until softened. Remove to a small bowl.
Add another 1/2 tbs of butter to the pan and heat until sizzling, swirling to coat the bottom of the pan.
Add half of the beaten egg mixture to the pan. Let set for 15 seconds and then begin lifting the edges one at time while tilting the pan towards the lifted edge, letting some of the egg run under the edge. Do this with a few more edges until the omelette is just set.
Quickly spread half of the filling ingredients evenly over the omelette, ending with a layer of cheese. Fold the omelette in half, lifting one side and folding it over the other. Let it warm for a minute and then slide onto a plate.