Mix the flour and salt together. Gradually add water, mixing until a smooth dough forms. If too wet, add a bit more flour; if too dry, add a bit more water. Let the dough rest for 10-15 minutes.
Crumble the sheep's cheese and finely chop the parsley. Mix them together with a small amount of oil.
Divide the dough into balls about the size of an orange. On a floured work surface, roll each ball very thin. Spread the cheese and parsley mixture on one half, add chopped onions if desired, fold the dough in half, and seal the edges by pressing them together with a fork.
Heat a skillet or pizza pan over medium-high heat. Fry the Börek on both sides without adding oil, until lightly golden.
Cut each Börek into small pieces and arrange on a serving plate. Drizzle melted butter over the top.