Melt 25g of the butter and fry the turnip, leeks, celery and carrot for about 5 minutes.
Add the beans, tomatoes, tomato puree and garlic.
Pour in hot stock.
Use pressure level 2.
close lid and bring to full pressure.
Cook for 25 minutes.
Lift the cooker away from the heat, and to cool at room temperature without altering the pressure selector.
When the pressure indicator drops down, open the lid.
Add the spaghetti and the remaining butter.
Bring to the boil, stirring occasionally and boil without the lid on for 15 minutes.
Serve immediately and sprinkle over grated cheese.