Blanch spinach leaves very quickly in boiling water and drain on absorbent paper. Set aside at room temperature.
Placevinegar and sugar in a small sauce pan over high heat and reduce for 10 minutes. Heat butter in another sauce pan over medium heat. Add onions, season with fine salt, and sauté for 4 or 5 minutes. Add vinegar mixture and continue to cook over high heat until thickened. Set aside in warm oven.
Poach apple balls in wine for 2 minutes. Drain and keep warm in the oven.
In a small sauté pan over high heat, sauté the foie gras slices until brown on both sides and soft in the center. Drain on absorbent paper and keep warm.
Arrange 5 spinach leaves on each plate. Place a flat mound of cooked onions in the center. Arrange 5 apple balls around the onions. Top the onions with a foie gras slice. Sprinkle with a few grains of fleur de sel and serve immediately.