Put egg yolks, 1 tablespoon lemon juice and chervil sprigs into the bowl of a small food processor. Process for 30 seconds or until combined. With processor still running, slowly add oil in a steady stream until a thick creamy sauce forms. Add warm water, 1 tablespoon at a time, if too thick. Transfer to a bowl.
Layer cucumber slices, tomato slices and onion rings on a serving platter. Season with salt and pepper.
Line a bamboo steam basket with a sheet of baking paper. Put chicken breast fillets in a single layer into the prepared basket. Cover with basket lid and place basket over a saucepan of simmering water. Cook for 10-12 minutes or until chicken is cooked through. Transfer chicken to a chopping board. Stand for 5 minutes. Thinly slice diagonally and place onto salad. Spoon over chervil aioli and serve.