Combine flour and salt in a bowl. Add butter and mix until crumbly.
Mix egg, cold water, and vinegar. Add to flour mixture and form a dough.
Knead gently for 2 minutes, wrap, and chill for 30 minutes.
Heat butter in skillet, sauté onions and peppers for 3 minutes.
Add beaten eggs, season, and scramble until soft. Stir in cheese.
Cool filling completely before assembling.
Roll out dough, cut circles, and add 2 tablespoon filling per circle.
Fold, seal with fork, and brush with egg wash.
Bake at 375°F (190°C) for 20–25 minutes until golden.
Cool for 5 minutes before serving.