Pickled red onions are easy to make and very delicious.
2 cups water
2 cups white wine vinegar
1 1/2 Tablespoons sugar
2 teaspoons of salt
I do not own a mandolin slicer, therefore, I slice my onions as thin as I can. This works perfectly fine. My husband actually likes the thicker onion slices.
Heat the water and vinegar, adding the sugar and salt to dissolve. Place the cut onions in a quart jar and after the water cools pour it over the onions. I like to add only peppercorns to my onions. You may like to add garlic or herbs, do whatever you and your family likes. Keep them in the refrigerator, they are good for use after only a few hours, but they only get better over time.
I keep this as an ongoing food in my refrigerator because we enjoy eating them on our salads.
When the onions have been eaten, I will add freshly sliced onions to the vinegar water. That vinegar water is concentrated and is still able to pickle satisfactorily, I will do this about two extra times. This is especially helpful when I am in a pinch for more onions as soon as possible.