Heat the butter and oil in the cooker. Saute the onion until soft but not brown, about 2 minute Stir in the rice, making sure to coat it thoroughly with the fat. Stir in 3-1/2 c of the stock (watch for sputtering oil).
Lock the lid in place and over high heat bring to hig pressure. Adjust the heat to maintain high pressure and cook for 6 minute Reduce pressure with a quick-release method (see below). Remove the lid, tilting it away from you to allow any excess steam to escape.
Taste the rice, and if it's not sufficiently cooked, add a bit more stock as you stir. Cook over medium heat until the additional liquid has been absorbed and the rice is desired consistency, another minute or two. When the rice is ready, stir in the Gruyere and Parmesan, add salt to taste and serve immediately.
NOTES: Adding some sort of fat is critical when cooking rice and grains so that the liquid/starch doesn't foam and plug up the steam vents. I'd also keep a close eye on it toward the end as I imagine that if you use less water the rice might dry out in the last minute or so. Also, Cooking Under Pressure is a nice reference book with a number of modern/gourmet- type recipes such as this one.
For better taste and texture, it's essential to use an Italian short-grain white rice such as Arborio, Carnaroli, or Maratelli. Traditionally the rice is not rinsed before cooking since the water would wash away starches that contribute to the velvety sauce enveloping each grain. The perfect risotto should be slightly soupy and properly chewy, with the rice offering just a pleasant resistance to the bite. For this reason, the pressure is always quick-released and the risotto must be served as soon as it is finished -- Leftover risotto can also be shaped into pancakes and warmed or pan-fried in a little butter, or heated in the microwave.
Using the basic formula of 3-1/2 to 4 c of liquid to 1-1/2 c Arborio rice you can create your own recipes and also use traditional recipes.
Although classic risottos usually contain wine, the above recipe(s) are flavorful without it.