Cut the Onion in Half LengthwiseYour onion now has two flat ends. That's good, because you can turn it so that it's resting on one of the flat ends and it won't roll around. You should have the other flat end facing up toward you.
Now place the blade of your knife across the center of the upward-facing flat end. The edge of the blade should intersect with the little bulls-eye in the center of the onion. Carefully cut the onion in half by slicing straight down through the center.
Note the way my thumb and forefinger form a sort of arch over the top of the knife. That lets me keep the onion nice and steady without getting my fingers in the way of the blade.