Food rings or cooking rings are simply metal rings without a top or bottom and they are used to present food in a round, compact form. You can use them to present the perfectly round fried egg or to present a portion of a stack of just about anything edible. They are great for serving individual portion of rice, for example. The only limitation to their usage is your imagination. If you cannot buy them locally, you can buy them online from Lakeland. See the blogroll.
You can usually buy them in a number of dimensions. I stick to one size — about 8 cm in diameter and about 4,5 cm in height. I find that this size is the most convenient and, frankly, for what they are, they are rather expensive.