So much can be said about adding garnish and about plating. One thing I try to do is to always integrate garnish in the dish. There's no such thing as a plate plus garnish, it has to be one.
In the picture you posted, I would encourage you to do another dish using a bell pepper cut like that. My suggestion would be to use it as a "source" from which interior your food starts flowing in a small stream, getting more and more wider further away. Try to plate quickly without hesitation adding just a bit at a time. This sequential adding food will stimulate your judgement when to stop adding while doing it and always keep in mind; less is more! A plate best looks like it's done nonchalantly but never as if it's thrown on the plate.
Also, take it as a general rule when using plates with no rim, to use only 1/3th or only 2/3th of the surface available. It has nothing to do with using so called blanc or negative space but so much more to do with a natural perception of harmony called Golden Ratio (I'm not going into that anymore).
When using garnish, use it as a complement to the dish, or to echo what's already in the dish, or to add a surprise. There's probably many more reasons why garnish is important. In summer you might add some fresh herbs to simply give it a fresh look and taste etc.
A few examples;
- Pasta verde with smoked salmon and salmon roe
If you look at the whole plate, you will notice that only 1/3th of the plate (in fact it's a bowl) is used. The roe is used as a garnish but it is complementary to the dish and it echoes the addition of smoked salmon. The whole plating is done very careful in the shortest time possible but it has to still look as if no effort has been done to plate it.
- Red beet with burnt feta, pistacchios and fresh oregano
Here, 2/3ths of the plate is used and it's all done very nonchalantly and very quick, keeping in mind to not cover more than the intended 2/3ths of the plate. The sprinkle of chopped pistacchios doesn't brake that rule at all, it simply emphasis this partition! Both pistacchios and oregano are used to complement the dish. Also, adding a crunchy element to a dish like nuts etc. makes a dish exiting!
- Pork chop with leeks in a gorgonzola sauce and polenta fries
Again a 2/3th occupation, so to speak, but a little off-center. When doing that, keep everything together instead of putting it here and there. Here I used an element of surprise; the polenta fries presented as a classic "pomme Pont-Neuf" presentation (classically only done with thick potato fries). Again, the crunchy polenta is added to make a variation on textures on the plate; remember, crunchy makes a dish exiting.
Try to cut all elements on the dish harmoniously to each other; plating starts at the "mise en place".
Keep on trying and let nobody stand in your way to experiment and to make a lot of mistakes, it's the only way to get skilled; get cracking. Remember, experience is the sum of our mistakes! Get your inspiration from others and steal with your eyes. Sooner or later you will discover some rules of your own or from others.
I steal everywhere and I have this Pinterest page to prove it; https://nl.pinterest.com/ChrisBelgium1/food/
Pinterest is the best source for looking at plating concepts imo!