Boil the egg in boiling water for about 8 minutes until hard, then quench. Wash and clean the beans and cut them in half or in thirds, depending on their size. Cook in boiling salted water for 6-8 minutes until al dente. Rinse immediately in cold water.
Mix stock and lemon juice. Add oil and season to taste with salt, pepper and sugar. Wash the tomatoes and cut them into halves or quarters depending on their size. Peel onion and cut into rings. Clean, wash and cut the lettuce into strips.
Drain the tuna. Mix salad ingredients, arrange on a plate. Peel the egg and cut into slices with an egg slicer. Drizzle with dressing and cover with egg slices. Serve with baguette bread.