Pro tips to make your life easier
To get the flavour amped up I use a combo of white wine vinegar, lemon and extra virgin olive oil as the dressing. Another tip is to separately soak both the red onion and apple in a little lemon juice too. It brings out the natural sweetness in both and also stops the apple from oxidising and turning brown. On the subject of apples, be sure to choose a crisp, crunchy apple like pink lady, honey crisp, fuji or golden delicious. These varieties work the best.
As well as oranges, i've made this dish using pear instead of apple which was a good option, just be sure to get crisp pears and not soft, ripe ones. I'll often change up the herbs and have used tarragon, which also has an anise flavour - I've also used oregano, marjoram and basil alongside the parsley. It's pretty versatile. And lastly, I'll sometimes replace red onion with white or spring onion.