Cauliflower & potato curry

Name: Cauliflower & potato curry
Prep time: 15 min
Cook time: 40 min
Total time: 55 min
Cuisine: Indian
Ingredients: 2 tbsp vegetable oil; 1 large onion chopped; large piece ginger grated; 3 garlic cloves finely chopped; ½ tsp turmeric; 1 tsp ground cumin; 1 tsp curry powder or to taste; 227g can chopped tomatoes; ½ tsp sugar; 1 cauliflower cut into florets; 2 potatoes cut into chunks; 1 small green chilli halved lengthways; squeeze lemon juice; handful coriander roughly chopped, to serve; naan bread and natural yogurt, to serve