Place stock in a medium saucepan. Smash the garlic with the back of a knife and add to the stock along with salt, pepper and herbs. Squeeze the lemon and add to stock. Bring stock to the boil then add chicken breasts. Poach gently for 4 minutes but do not allow to stock to come back to the boil. Remove saucepan from heat and allow chicken to sit in stock until just cooked through, about 8-10 minutes.
For the gribiche, dice or coarsely grate the eggs, place in a mixing bowl and add the remaining ingredients. (If using runny soft-boiled eggs, smash in a bowl with a whisk, whisk in the oil until emulsified, then add remaining ingredients.) Stir through until well combined, adjust seasoning to taste.
Remove chicken from stock. Peel off skin and discard. Slice meat on a diagonal into thick pieces. Place salad leaves and herbs in a bowl and dress with extra virgin olive oil, lemon juice and seasoning.
Place a handful of salad over four plates, add chicken, garnish with gribiche. Serve immediately.