Preheat the oven to 350F.
Peel and thinly slice the onions. Try to cut them as thin as possible, a mandolin slicer, sharp knife or food processor blade attachment works nicely.
Heat the olive oil in a large skillet over medium-high heat.
Add the thinly sliced onions and reduce the heat down to medium, stirring occasionally until the onions turn a deep brown, around 30-45 minutes. This part can take a little while, so be patient and resist turning the heat up, the flavor of burnt onions cannot be saved.
Once the onions are caramelized, add in the Arborio rice and salt, and toast the rice for 3-4 minutes, stirring constantly.
Pour the rice and onion mixture into a 8"x12" baking dish or an oven proof Dutch oven.
Working quickly, pour the hot beef broth over the rice and onion mixture and give the rice a stir.
Gently shake the baking dish to level out the rice and cover the dish tightly with foil. If using a Dutch oven, cover with the lid.
Bake for 30 minutes until the rice is tender and the broth has been all but absorbed.
Stir in the parmesan cheese until a creamy texture is formed. If the rice seems a little too dry, try adding in a small ladleful of hot beef broth while stirring until the preferred texture is achieved.
Top with parsley and serve piping hot.