Whisk mustard, salt, sugar, and vinegar, then marinate 1cm cucumber slices for at least 15 minutes.
Trim duck breasts of fat and sinew and season with salt and pepper.
Sauté duck in oil over medium-high heat for four minutes per side until browned but pink inside.
Boil soy sauce, sake, and sugar in the pan until syrupy, then glaze and slice the duck.
Heat oil for deep frying in a heavy pan until it sizzles.
Lightly mix egg yolk and iced water, then briefly stir in flour to keep the batter lumpy.
Dust celery leaves in flour, dip in batter, and fry for three minutes until golden.
Serve sliced duck with the marinated cucumber and garnish with tempura celery leaves.