Heat the oven to 220°C/410°F. Bring a pot of water to the boil and add the sweet potato, cook for 10 minutes then drain and add to a bowl. Mash with a fork.
Cook the pasta according to package instructions. While that's happening melt the butter in another saucepan and add the flour, whisk together to create a paste then slowly add the milk and continue to whisk until smooth and thickened. Add the mustard.
Reserve a little pasta water then drain the pasta and add it to the sweet potato with the gouda, cheddar and white sauce. Stir together, adding a little pasta water if needed. Tip pasta mixture into a baking dish and cook for half an hour.
Nutrition information is an estimate and will vary depending on the ingredients used.