Yes you can cook onions in cast iron.
Cast iron skillets can be used to cook onions in and they work great to cook onions.
Just be sure to add some cooking oil to the cast iron skillet before placing the onions in the cast iron skillet.
Also be sure to stir the onions around in the cast iron skillet so they don't burn or stick to the pan.
If you want to caramelize onions in a cast iron skillet you can do so as well.
How to caramelize onions in a cast iron skillet.
Heat the oil in a 12-inch cast iron skillet over medium heat.
Add the onions and sauté for 5 minutes or until starting to soften.
Add the salt, reduce the heat to medium low and cook for 50 to 80 minutes, stirring every few minutes, or until the onions are very soft, golden brown, and caramelized.
If you just want to simply cook the onions then it takes a few minutes to 5 to 10 minutes of cooking.
Keep an eye on the onions as they are cooking so they don't burn.