When the onions are ready, put a layer of them on two slices of the bread, then top with a layer of sliced gruyere.
Toast sandwiches on a panini press or similar type of grill until cheese is melted and bread is toasty.
Notes
You can easily adjust this recipe to make any quantity of sandwiches. You need one-half of a large white onion per sandwich.
Do not add any extra butter or cooking oil to the sandwiches when toasting them. The onions have plenty of butter on them which will seep into the bread as it cooks. Any additional oil or butter will result in a very greasy sandwich. Which is fine if you like that, but I'm not responsible.
Recipe by Sprung At Last at https://sprungatlast.com/2013/01/french-onion-soup-grilled-cheese/