1. Heat your oven to 180C/Gas Mark 4. Pierce your sweet potatoes a few times with the end of a sharp knife or fork and bake for 45 minutes, or until cooked through. Remove from the oven leave to cool slightly.
2. Slice in half and remove the insides. Place the orange centre on a baking tray, chop roughly and leave to cool completely.
3. Once cooled, put in a large bowl and sift the flour over the potato. Use your hands to combine.
4. Add the egg yolks, tarragon, nutmeg and some salt and pepper then continue to combine with your hands or a wooden spoon.
5. Roll into a flat sausage shape then cut into small 2cm wide parcels. If you want, and to help the sauce cling to the gnocchi, press a fork lightly into the sides of the gnocchi to give it ridges.
6. Bring a pan of salted water to the boil then cook the gnocchi for 2 minutes before lifting them carefully out of the water and leaving to cool.
7. Heat the olive oil in a large frying pan then fry the garlic for 10 seconds before adding the chopped tomatoes. Bring to the boil and simmer on a high heat until the tomatoes reduce to a thick sauce. Season with salt and pepper and add the unsalted butter.
8. While you are waiting for your tomato sauce to thicken, heat the remaining butter in a separate pan and gently fry the gnocchi on each side until golden brown.
9. Serve with the tomato sauce either on the side or on top of the gnocchi and cover both with the grated parmesan.