Core and roughly chop the peppers and place them in the oven on gas mark 5 (375F/191C). Check on them after half an hour. Give them a stir and move them down a little if necessary. 

When you're ready to make the meal, boil the pasta according to the instructions.

While it's cooking blend the roast peppers and cannellini beans in a food processor. When nearly smooth, add the stock, garlic and black pepper and blend.

Make the cornflour into a paste with a little bit of water or stock, add and blend.

When the pasta is tender drain and stir in the sauce. Place back on the heat, stirring frequently, until it is heated through and thickened.