Put 2 Tbsp butter, onion and garlic in the rice cooker.
Turn the rice cooker to Cook. when the butter is melted let it "cook" an additional 2 mins.
Add the arborio rice, stir and cook 10 mins, until the rice is mostly clear with a small white center in each grain.
Add broth, salt and pepper, stir and close the lid and reset cooking time, Because there is so much liquid it will take longer to cook than a normal setting so leave yourself extra time.
When the machine is done let sit for at least 10 mins or until your ready to serve then stir in the last 1 Tbsp of butter and parmesan.