Start by mixing all the dressing ingredients together in a bowl. Taste, and add more salt if necessary. Set aside.
Halve, the thinly slice the onion. Add to a large bowl.
Halve the tomatillos, then slice into 3-4 wedges, depending on their size. Add to the bowl.
Pile your basil leaves onto each other and roll into a chiffonade. Then thinly slice them. Add to the bowl.
Finely chop your chillies and garlic, and add to the bowl.
Pour the dressing all over the salsa and mix thorough, either with your clean hands or with 2 large wooden spoons.
Cover with a clingfilm or plate and leave to stand in the fridge for 10 minutes before serving. This will allow the onions to mellow slightly, and the flavours to develop. But this step is optional, you can serve it immediately if you like. I do, as I like my onions!
Notes
Total time doesn't take into account the 10-minute optional resting time.