I was watching the finale of Amazing Race last week, and one of the final tasks was to dice 50 onions. If any of the contestants had seen a tutorial such as this one, they would be owing me some cash.

The first step to learning about dicing any vegetable, is understanding the concept of having vegetables cut to sizes in relation to cooking time. That is, you want your items to cook at a similar rate. So if you have carrots, celery, onion, you know that the carrot will take longer, so perhaps you want it slightly smaller in size than the onion and celery. Likewise, if you only are using onion, you want them to be fairly uniform in size, so that it cooks at a consistent rate. That can be said for anything, vegetables, chunks of meat, you name it.
The first step is to remove the ends, as little as possible, and remove the outer "paper" and "skin".

The next step is to cut the onion in half, through the "poles". What I mean, is if you consider the onion a globe, you want to slice from the North Pole to South Pole, not around the equator.
This gives you a flat surface, which is always imperative to safe knife handling. You want to always work with flat surfaces, versus round, which can result in injury (I have a gruesome story of me and a rutabaga).

So now you are ready for the cuts that make the dice so easy. The first step is to make two or three slices perpendicular to your cutting board. You only want to slice 80% or so, you want to leave the end together. Again, this keeps the onion together, making your job easier.

As I do in the photos, you want to keep your fingers atop the onion. These are not precise cuts, so do not worry about how flat the cut, or how far apart the two or three cuts are. The cut is just segmenting off the layers.

The next cut is where you determine how big the dice is going to be. If you are making a stew, and only need a large dice, as the onion will cook for a long time, you will make fewer cuts. A sauce, or tuna salad, you want a fine dice, so you will make many slices. These cuts are going to be perpendicular to the last set of cuts made. starting at the left of the onion, start slicing the onion. Again, be sure not to slice all the way through the onion, we still want a "butt" holding the whole thing together. Continue making slices the remainder of the onion. Again, the amount is determined by how fine a dice you desire.

The onion at this point is going to sort of like an accordian. The final step is to slice downward opposite the second set of cuts. A cut very much like if you were simply slicing the onion.Starting at the end opposite the "butt" that is still holding our onion together. The size of this cut is also in relation to the size of the dice, and would be in similar amount as the second set of cuts.

This cut should start resulting in the final diced onion. Ultimately, you will wind up with a small butt that you can either do a simple dice on, or, through in a bag for making stocks.
With about 18-24 cuts to each half of a medium sized onion, you should have a fairly consistent sized diced onion.

This isn't a technique that will be mastered with the first onion, nor with the second. But with a sharp knife, patience, and practice, you will find your self spending less time doing the little tasks, and more time enjoying new dishes.