In a mortar, grind the anchovy fillet and garlic. Add the mustard and grind again. Add the vinegar and grind. Add the oil in a stream, continuing to grind with the pestle to emulsify. Season with salt and pepper as needed. Set aside.
Lightly oil the corn on the cob and grill them for a few minutes on all sides in a BBQ at high heat. Leave to cool and remove the kernels from the ears using a sharp knife.
Cut the baby potatoes and the blanched beans in 2, divide in a serving dish with the corn kernels and watercress. Add the egg quarters.
Pour in the dressing and garnish with a pinch of chili peppers. Serve the salad warm or cold.
The little extra
If corn season is over, you can use frozen corn (not thawed) and grill it with a little oil in a cast iron skillet at high heat.