Heat a griddle pan. Toss the peaches with olive oil and some ground black pepper. Grill on their cut sides for 1-2 mins.
Place on a baking sheet and bake in the oven, 200°C (400 F) for 10 minutes or until soft.
Arrange Mache, onion and radishes on a plate. Finish with peaches and walnuts on top, scatter with the feta. Drizzle with olive oil and season with salt and pepper. Eat straight away.