Green chicken salad

Name: Green chicken salad
Ingredients: 1 small red onion, finely chopped; 1 lemon, juiced; 1 small rotisserie chicken, around 250g meat, picked and shredded; 4 sticks celery, finely chopped; 4 spring onions, finely sliced; ½ large cucumber, deseeded and finely chopped; 6 cornichons, roughly chopped; 1 green pepper, deseeded and cut into 1/2cm cubes; large handful of soft herbs (we used dill and parsley), roughly chopped; toasted sourdough, to serve; 50g natural yogurt; 2 tsp Dijon mustard; 50g mayonnaise; 50ml olive oil