Slice the truffles into 2 mm thick pieces. Melt a knob of butter in a sauté pan over medium heat. Fry the slices for 3 minutes per side, seasoning with salt and pepper. Set aside on a cloth.
Discard the cooking butter, deglaze the pan with sherry vinegar, and reduce to a glaze. Add the Cassis wine and reduce by one-third. Set aside.
Brush the tops of the puff pastry rectangles with beaten egg. Place on a baking sheet and bake in a preheated oven at 200°C (th. 6) for about 15 minutes until golden. Set aside.
Slice the foie gras into escalopes, season both sides with salt and pepper. Heat a nonstick skillet until hot; sear the escalopes for 3 minutes per side. Remove and pat dry with paper towels. Set aside.
Split the puff pastries horizontally, removing the tops. Layer foie gras slices and truffle slices alternately in the bases.
Reheat the sauce over low heat and stir in the cream. Warm the filled puff pastry bases with tops in a low oven (th. 2, about 80°C) for 3 minutes.
Add the remaining butter to the sauce, swirling the pan until fully emulsified. Adjust seasoning to taste.
Remove puff pastries from the oven, lift the tops, spoon sauce over the filling, sprinkle with chopped chervil, and replace the tops. Serve immediately.
Expert Tip: If Cassis white wine is unavailable, substitute a white Graves. For a slightly acidic sauce, add a pinch of caster sugar.
Chef's Note: This luxurious dish is indulgent and pricey—scaled for two. Ensure the foie gras is hot inside by pressing the center; cook longer if needed.
Suggested pairing: White Cassis wine (Clos-Sainte-Magdeleine).
