My vegetable steamers are essentially berry bowls with lids. The holes in a steamer are generally lower than those in a berry bowl. You may take a saucepan slightly smaller in width than the steamer and fill it with about an inch of water, so the water does not touch the steamer. Place the steamer on the saucepan, bring the water to a boil and steam your vegetables for 10-20 minutes, depending on the vegetable.
Stoneware is safe to use in conventional ovens, convection ovens, microwave ovens, and is freezer-safe and dishwasher safe.