Step 1 Spray a large non-stick frying pan (which has a lid) with olive oil and set over medium heat. Add chopped onion, garlic and celery; cook, stirring occasionally, for 3 minutes, or until fragrant.
Step 2 Increase heat to medium–high. Add lamb mince to pan and cook, stirring with a wooden spoon to break up mince, for 5 minutes, or until browned. Stir tomato paste into mince and cook for 1 minute.
Step 3 Add sweet potato, tomatoes and beef stock to pan; cover and bring to the boil. Reduce heat. Simmer mixture for 20 minutes, or until potato is tender. Stir baby spinach and peas into mixture; cook until spinach wilts. Leave mixture to cool slightly.
Step 4 Preheat oven to 200°C. Spray 4 x 1 1/2-cup capacity (10cm wide) heatproof dishes or bowls with olive oil. Spoon lamb mixture into dishes. Lay pastry sheet on a work surface to thaw, then cut out 4 x 12cm rounds. Top pie filling in each dish with a pastry round, pressing excess pastry over edge. Use a sharp knife to score centre of each pastry lid with two small slits and brush lightly with milk.
Step 5 Bake pies for 20–25 minutes, or until pastry is puffed and golden, then serve with salad leaves.