Lamb rogan josh with rosti

Name: Lamb rogan josh with rosti
Ingredients: 2 medium potatoes (400g); 40 gram medium potatoes (400g); 2 teaspoon vegetable oil; 1 medium brown onion (150g), chopped finely; 1 clove garlic, crushed; 1 centimetre piece fresh ginger (5g), grated; 600 gram lamb backstrap, chopped finely; 2 tablespoon rogan josh curry paste; 2 medium tomatoes (300g), seeded, chopped finely; 2 tablespoon lemon juice; 2 tablespoon finely chopped fresh coriander