Put a pot on the stove to heat the stock (or you could heat it in a jug in the microwave), I use this opportunity to cook my peas. Then I dig them out with a slotted spoon and set aside. Put the lid on the stock to keep warm
Put a high sided heavy pot onto the stove on medium. Add the olive oil and the onion and celery and soften for 2 minutes. Add the rice and toss in the oil till warm. Begin to add the stock 1 cup at a time. Turn the stove down a little so that the liquid doesn't evaporate too quickly. Stir occasionally.
Put a non stick fry pan on to cook the sausages. Add the extra oil and let them cook on medium till done.
Meanwhile add the stock slowly, cup by cup. You can add the tomato paste after the second or third cup. (Don't let the rice dry up and fry)
By the time all of the 1 litre of a stock ( approx 4 cups) is added the rice should be cooked through with just a small amount of firmness in the grain. All rice is very different so add a little more stock if you want your rice more cooked.
Add 1/2 of the cheese and the butter ( if using) stir through and put on the lid and sit for several minutes while you cut the sausages into quarters .
Serve the risotto with sausages some snipped chives a few peas and extra cheese,