Keema Pie with rosti topping

Name: Keema Pie with rosti topping
Ingredients: 3 Tablespoons vegetable oil; 1 large onion, finely chopped; 4 garlic cloves, finely chopped; Knob of fresh ginger, grated; 2 long green chilies, deseeded and finely chopped; 2 teaspoon cumin seeds; 2 teaspoon coriander seeds; ½ teaspoon ground turmeric; ½ teaspoon hot chili powder; 500g beef mince; 300ml beef stock (I used Knorr Touch of Taste); 400g tin peeled plum tomatoes, discard the juice; 200g spinach; 1 Tablespoon lemon juice; 1 teaspoon garam masala; 1 medium potato, coarsely grated; 1 medium sweet potato, coarsely grated; 1 medium onion, finely sliced