Step 1: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
Step 2: Gently unroll the cold puff pastry sheet onto a lightly floured surface or directly onto the parchment paper. If needed, lightly roll it out to an even thickness, about 1/8 inch.
Step 3: Using a sharp knife or pastry wheel, cut the puff pastry into 12-16 equal squares or triangles, depending on your desired shape and size.
Step 4: Cut the chilled foie gras into small cubes or slices, roughly 1/2 inch in size. You'll need one piece per pastry square/triangle.
Step 5: Place one piece of foie gras in the center of each pastry square/triangle. If using squares, bring opposite corners together to form a small packet or fold into a triangle. If using triangles, roll them up from the wide base towards the point, gently tucking the foie gras inside.
Step 6: Transfer the assembled croissants to the prepared baking sheet. Lightly beat the egg in a small bowl to create an egg wash. Brush the tops of each croissant with the egg wash for a golden finish. Sprinkle with flaky sea salt, if desired.
Step 7: Bake for 12-15 minutes, or until the croissants are puffed, golden brown, and the foie gras is warmed through and slightly softened. Keep a close eye on them to prevent over-browning.
Step 8: Remove from the oven and serve immediately. They are best enjoyed warm, ideally with a small dollop of fig jam or onion confit on the side.