Chop the garlic and onion finely. Heat the oil in a pan over medium heat and sauté the garlic and onion for 3-5 minutes until translucent.
Cut the chicken breast into small pieces (or shred leftover roast chicken) and add to the pan along with the frozen vegetables, sour cream, mustard, and grated cheese. Stir until combined and heated through, about 5-7 minutes. Remove from heat and let cool for about 10 minutes.
Preheat the oven to 220°C (428°F).
Divide the puff pastry into 8 pieces. For each piece, spoon filling onto one half, leaving about 1 cm clear around the edges. Beat the egg and brush it on the pastry edges. Fold the pastry in half and press the edges firmly to seal.
Place the pastries on a baking sheet and bake at 220°C (428°F) for 15 minutes until golden brown.