When you cook an egg over hard, you're essentially cooking it until the whites are firm and the yolk is cooked through. This is opposed to other methods like sunny side up or scrambled, where the yolk is left runny. To cook an egg over hard, start by heating up some oil in a pan. Then, crack the egg into the pan and let it cook for a minute or two before flipping it over. Cook for another minute or two on the other side, or until the egg is cooked to your liking. Serve hot!