It is very easy to slice some thick onion pieces. This, however, may not be the best option for the type of meal you are making. You may want a thin slice of onion on a hamburger. You may also want to add a little flavor to your salad without it becoming overwhelming. In this case, thinly sliced onions would work better. Slicing onions thinly takes a little practice, but it isn't that hard if you have the time.
Cut off the top of the root from the onion. You will then be able to peel back the skin and discard it.
Use a chef's knife to cut your onion in half. Then place the onion on a cutting board with the cut side facing down. The cut side is the flat side.
Take one half and place it in front of you with the root side facing left if you are right-handed. If you are left-handed you will have the root facing the right side.
Grip your chef's knife with your last three fingers. Wrap them around the handle of the knife. Place your thumb and pointer finger just above the handle as if you are pinching the blade.
Place the tip of the knife up against the cutting board and the blade up against the side of the onion.
Hold the onion in place with your opposite hand. Bend your fingers so that your knuckles are up against the blade of the knife.
Move the knife down and forward at the same time. This will allow you to cut a thin slice, versus just moving the knife down and forcing the onion to chop in to a thick piece.
Keep your windows open so that the onion doesn't make you cry.
Practice cutting with the knife first so that you get comfortable with it. This way you don't accidentally cut your knuckles.