Marinated onions are divine. You can serve them many ways. You can cook them more ways. Main thing: good oil and I like a butter combination. You can cook them until they are totally brown and they will be sweet as honey. I like to cut mine in rings so you can have a bit of their total flavor, rather than chopping them up.
Carmelizing is what you are doing. It really concentrates the flavor of an onion. You can put them in your mashed potatoes and since St. Patty's Day is almost here, you can put them in your colcannon.
Just experiment. Go for it. Sauteed or carmelized onions are another of the many gifts from God.
If all you want to do is marinate them, just separate the rings, put them in oil and vinegar and whatever spices you like and let them stay in the frig for a few hours. Try making a marinated tomato, onion and anchove salad. It will be a hit. Even better if you can find Torpedo Onions!