How to keep onions from sinking in a salad

This is physics.

There's an effect called: "Granular convection" It's a phenomenon where if you have materials of different sizes in a container and vibrate or shake them, that the largest objects will move to the top and the smallest to the bottom.

To keep the onion from sinking, you should make the chopped onion pieces bigger relative to the other ingredients in the salad.

As you can see in JBentley's answer, in a salad where all ingredients are evenly chopped you won't get the sinking effect.

I don't think the issue here is dicing vs slicing the onions. The reason why the latter appears to present better in this answer is in my opinion because of the size relative to the other ingredients. In the left hand image, the diced onions are much smaller than the tomato and cucumber, whereas the sliced onion on the right is a closer match in size. This also explains why the onion sinks in the left image - being smaller than the other ingredients, it can make it's way through the gaps more easily to the bottom.

This leads to an alternative solution: dice the other ingredients to match the size of the onion. Take for example a salad shirazi (a classic Persian salad):

As you can see, the onion is nice and evenly spread throughout the other ingredients (no sinking) and in my opinion presents rather well.

Also it's worth being aware that this is highly subjective. I personally find smaller chunks more attractive than bigger in general. I also find it improves the taste, as the ingredients blend better together while chewing and you experience the salad as more of a "whole". With bigger chunks, you tend to taste "all tomato" followed by "all cucumber" etc.

I found that slices float better and present better. They don't take much longer to cut and you don't need much onion for a salad.

Slices on the right

Thick slices can be cut to taste. You want some to sink. I like them for a little crunch. They're easier to pick out if anyone doesn't like onion at all.

In some cases thin slices might be better, though. If your salad is lettuce-based, thick slices may still tend to sink, while thinner slices will cling to the lettuce a bit better. This may also be a good idea if anyone eating the salad doesn't like larger chunks of onion. A down side to thin is that it can overpower the presentation.

There is no law of nature that requires salad to be served in a bowl. In fact I rarely see it served in a bowl: I'm used to seeing it served on a plate. You can toss together the leafy components, dressing, and any other ingredients you want to mix well, plate them, and then sprinkle the onion on top and serve.

I recently discovered pickled red onions (and I've added the recipe I use on my website), and they're really easy to make.

I say this because they're an excellent addition to any tossed salad, and because they're saturated in vinegar, they're not rigid, so leaving full rings isn't a problem like raw onions.

Also, because they're larger pieces, they don't settle in a salad.

Similarly I avoid using chopped carrots, favoring shredded or matchstick carrots because they have the same issue of being too small that the end of the salad is all carrot.

This is also known as the "Brazil Nut Effect".

There is a very simple way to counter that porblem if you are going to dress a salad in a bowl. You can cut any ingredients in the size you want, so you can dice oignons.

Before serving, when you added everything, just take a tool like a spoon or a wooden spoon and "incorporate" everything delicately. All your ingredients will be mixed up without any harm and therefore you will enjoy a delicious mix of your salad.

This action, "incorporate', comes from pastry cooks who use that to incorpore ingredients that are heavier than others.

just like in this video https://www.youtube.com/watch?v=UfqBYkNDRSg

This is simple. We can chopping an onion with loop chopping.