Step 1

Preheat the oven to 400 degrees F. Roll out the puff pastry into an 11-inch square on a sheet of parchment paper. Transfer the dough on the parchment to a baking sheet. Beat the egg with a splash of heavy cream. Brush a 1-inch border of the egg wash around the edges of the dough, then fold in each about 1/2 inch to make a thin border and press to seal. Prick the center of the dough all over with a fork, then brush all over with the egg wash.

Step 2

Bake the dough until crisp and golden brown, about 25 minutes. Prick the center of the dough with a paring knife and gently press with the bottom of a measuring cup to flatten, if necessary. Remove to a rack and let cool.

Step 3

Meanwhile, soak the sliced radishes in a large bowl of ice water until crisp and just starting to curl, about 15 minutes; drain and pat dry with paper towels.

Step 4

Mix the garlic-herb cheese spread and heavy cream in a medium bowl until smooth. Whisk the vinegar, tarragon, parsley and a pinch of kosher salt in a large bowl. Add the radishes and smoked salmon and toss to coat.

Step 5

Spread the cheese mixture on the cooled crust, then pile the radishes and smoked salmon on top. Sprinkle with flaky sea salt.