Heat a large pan over medium-high heat. Add the chopped onion and minced meat and cook, stirring frequently, for about 5 minutes until the meat is browned and crumbly. Season with salt and pepper to taste. Stir in the sheep's cheese cut into small cubes until combined.
Place the defrosted puff pastry on a work surface. Distribute the minced meat mixture along the center and fold the pastry to form a pocket or bag shape, pressing the edges together.
Coat the sealed edges with egg white and press firmly to close. Use a fork to press a decorative pattern around the edge of each pocket.
Brush the pockets all over with egg yolk. Sprinkle with rosemary and use a fork to poke several small holes in the pastry surface.
Place the pockets on a baking sheet. Bake at 180°C (356°F) for about 30 minutes until the pastry is golden brown and crispy.