Poach the chicken by placing the breast fillets in a pot large enough to hold them in a single layer. Add water until it covers them by a couple of centimetres. Add salt, peppercorns, a slice of lemon, and a few mint leaves.
Bring the water to a boil, then reduce the heat to low. Simmer for about 10 minutes. Turn off the heat and cover the pot. Let the chicken sit in the warm water for another 10 to 15 minutes. Drain and leave to cool. Slice the chicken and set aside.
While the chicken cools, make the mint sauce. Combine the mint leaves, salt, olive oil, and lemon juice in a blender or food processor. Blend into a smooth paste.
Stir the mint paste into the mayonnaise until smooth and creamy.
Add the sliced chicken to the mint mayonnaise and mix until evenly coated. Refrigerate overnight before serving.
Recipe Notes
For the best flavour, serve the poached chicken and mint sauce cold or at room temperature with a tomato and mint salad.