Beat in 1/3 cup flour with oil in a medium bowl with wooden spoon.
Add yogurt & mix well.
Stir in 1/3 cup more flour.
Sift cornstarch , baking soda & salt then stir into flour mixture.
Gradually stir in enough remaining flour to make a soft but not sticky dough.
Lightly dust with flour then cover with a kitchen towel & allow to rest at room temp, 1-1 1/2 hrs.
If using feta, soak in water 10 mins. drain & crumble.
In a medium bowl mix feta or roquefort & mozarella with your egg.
Shape cheeses into 2 medium balls & set aside.
Divide dough into 2 portions then roll each into a ball.
On floured surface flatten each ball into a 7" round.
With floured hands gently rotate dough & pull into a 10" circle, 1/4" thick.
Be careful not to tear your dough.
Pat each cheese ball into a 5" circle on center of dough.
Gently pull up edges of dough pleating & pinching to enclose cheese.
Pat into a 7" round.
Heat a large cast iron skillet over low heat for 3 mins.
Lighly film skillet with melted butter.
Place bread seam side up into skillet.
Cover then cook for 12 mins. over very low heat, shaking pan ocassionally.
Uncover then flip bread over, cover & cook over low heat until deep gholden brown color is westablished.
Lightly brush top of bread with butter & allow bread to stand for 5 mins.
Use a serrated knife to cut bread & serve warm.