Peel and chop the onion, stewing it in a pan with one tablespoon of olive oil until glassy. Add the rice to the onion and stir until all the rice grains are covered with a layer of olive oil. Cook the rice until glassy, and then add the champagne. Heat the stock in a saucepan, and then add the hot stock little by little to the rice, allowing the rice to absorb the liquid each time. Stir the rice regularly. Repeat this until all stock is used, which will take about 15 to 20 minutes. Taste the rice to see if it is done and if not, add an extra cup of stock, allowing the rice to absorb it. Stir the butter into the risotto and season with salt and pepper. Let the risotto sit for 5 minutes with a lid on the pan. While the risotto sits, place the prawns on an oven sheet and bake them at 450°F / 230°C until they are pink. Divide the risotto into 4 bowls, add a prawn to each bowl and add a piece of watercress as a finishing touch. Serve hot.