Make the puff pastry crescents: Lay out the puff pastry dough. Cut out 16 crescent shapes from the dough. Brush with egg white and bake in a preheated 375-degree oven for 15 minutes, until golden.

Make the stuffed chicken: Arrange the chicken breasts on a cutting board and cut them in half. Remove the fillet strip from each breast, discard the white tendon, and grind the fillets in a food processor. Add the ice, blend until the ice is absorbed, and with the motor running add the cream. Add the spinach, salt, fennel seeds, lemon rind, nutmeg, and pepper and blend the filling well, scraping down the sides.

Cut a slit in each chicken breast for the filling. Spoon about 3 tablespoons of the spinach filling into each pocket. Close with toothpicks. The chicken may be prepared up to this point 24 hours in advance and kept covered and chilled.

Place chicken in a baking pan. Add the white wine and enough chicken stock to barely cover the chicken. Add salt and pepper. Bake, covered, in a preheated 375-degree oven for 45 minutes, until the chicken is cooked through.

Make the sauce: In a saucepan, melt the butter. Blend in the flour and cook, stirring, for 2 minutes. Remove from the heat and whisk in the boiling chicken stock. Bring the sauce to a boil for 1 minute, stirring, until it thickens. Return to the heat and let simmer. Beat in cream by the spoonful, then the softened truffle butter a tablespoon at a time. Season with salt, pepper, and lemon juice to taste.

Remove chicken from the poaching liquid. Serve with the sauce and the puff pastry crescents.