Don't get stressed or cry about it. Don't try and cut like a professional chef if you don't really want to. Just a quick "rainbow cut" and sauté, fry or mix it into any recipe.
Here's how it works...
You begin with a whole onion. Cut off both ends and then lay the onion on one of the two flat sides and cut through the middle of it. Now, you can lay the most recent cut side flat as the picture here on the left shows.
Also a HUGE tip, place your onion (or ANY ingredient) at a 45 degree angle, also as seen in the photo to the left. This allows you to hold your knife right out in front of you, keeping your body straight with your belly against the cutting board. Your shoulders stay square to the board and you don't have to twist your body or move for the ingredient. The ingredient moves for you!
Next you will cut lengthwise through the onion a few times making little rainbows as seen below.
This one is cut with thicker "rainbows"
While...
This one is cut with thinner ones....
How thick or thin you slice the rainbows, helps make a smaller or bigger "diced" end product.
But no matter how thick or thin, rotate them back up to a 45 degree angle as seen below and...
cut through each of the rainbows lengthwise and it instantly reveals your size of "diced onions".
Only if I am making super fancy food, do I ever dice an onion in a professional/restaurant way anymore. My kids wouldn't notice the difference anyway.