Thaw your frozen spinach in the microwave or in a bath of warm water.
Separate whites from yolks. Save whites and add your yolks to thawed spinach.
Add salt, cheese, flour, and milk, mix well until combined.
Beat your white to soft peaks.
Add 1/3 of your whites into the spinach mix, beat until well combined.
Add the rest of the whites mixing carefully, don´t overmix, you want the air from the whites to stay airy!
Dollop a good tablespoon of olive oil into a ramekin and coat well. Add the souffle mixture into the ramekin.
Bake until soufflé is puffed and golden on top. The center should look set but the center moves slightly when dish is shaken gently. Takes about 25 minutes (do not open oven door during first 20 minutes). Serve immediately.