I always have onion salad with my currys and make it like this:
1 Onion(chop how you like it every BIR i've been to is different)
1 Tablespoon lemon juice
1 Teaspoon salt(or to taste)
1 Teaspoon of mint sauce
1 Teaspoon chopped fresh coriander
Mix together in a bowl, cover with cling film and leave in the fridge.
You can also add some chopped tom's and cucumber if you wanted.