Lightly wilt the buckwheat grains together with water with the lid closed for 10 min. Drain in a sieve, cool and drain well.
Cut carrots and celery into centimeter cubes, mix on the spot with the juice of one lemon. Steam in a steamer strainer over boiling hot water until just cooked, 8 to 10 min.
Meanwhile, for the dressing, crumble the Roquefort and place in a tall container with the remaining ingredients. Blend with a hand blender. Mix with the still warm vegetable cubes.
Heat the oil in a frying pan and roast the buckwheat grains for 5 to 8 minutes, turning them until crispy. Season with a little herb salt.
Spread the lettuce leaves on plates. Spread the vegetable salad evenly on it, sprinkle the buckwheat grains on top and garnish with cress sprigs.
Tip: Use creamy natural yogurt for an even finer result!