Vietnamese lamb shanks with sweet potatoes

2 tablespoons Ground Nut Oil 4 Lamb Shanks 2 Onion 2 tbs chopped Ginger 3 sliced thinly Garlic Clove 1 chopped Red Chilli 1 sliced Red Chilli 1 tablespoon Brown Sugar 1 tsp Brown Sugar 3 Star Anise 2 Lemongrass Stalks 1L Lamb Stock 1 ½ tbsp Tomato Puree 4 Sweet Potatoes 2 tablespoons Fish Sauce Juice of 2 Lime Handful Mint Handful Basil Leaves