Yes you can boil onions in water.
Just peel the onions and cut the onions up and then add them to the boiling water.
Boiling the onions takes around 10 to 20 minutes.
Get the water boiling and then reduce it to a simmer to allow it to slow boil and prevent the water from boiling over the pan.
You can boil onions and just about any other foods.
Just like potatoes, cucumbers, etc you can boil onions.
I like to boil onions and they are easier to chew and digest for me.