Prepare crust, if store bought look at directions day before as you will need to defrost overnight most likely. Needs to be chill when preparing but not frozen.
Preheat oven to 375F
Heat olive oil in large skillet, over medium heat.
Add the mushrooms, onion, and salt. Cook, stirring occasionally, until veggies soft and golden, 8-10 minutes.
Add the spinach and stir until wilts - about 4 minutes.
Place the eggs and half & half in small bowl, season with pepper to taste and whisk to combine.
Spoon the spinach mixture over the bottom of the chilled pie crust and sprinkle feta on top. Pour the egg mixture into the the crust.
Place on baking sheet in oven and bake until puffed and golden, 40-50 minutes. Let cool and serve warm or at room temp.