In a bowl, mix flour, salt, and herbs. Add cold butter and work it in with fingers or a pastry cutter until it looks like coarse crumbs. Stir in the grated Gruyère. Drizzle in cold water 1 tbsp at a time until the dough just comes together. Flatten into a disc, wrap in plastic, and chill 30 mins.
In a small bowl, combine cream cheese, with chives, dill, capers, lemon zest, and a little black pepper. Set aside.
On a lightly floured surface, roll chilled dough into a rough 10–12-inch circle. Transfer to parchment-lined baking sheet.
Spread the cream cheese mixture in the center, leaving a 2-inch border. Sprinkle the shredded Gruyère just over the top. Scatter flaked salmon evenly.
Gently fold the crust over the filling, pleating as you go. Brush the crust with the egg yolk + Dijon wash.
Bake in the oven preheated to 400°F (200°C) for 30–35 mins, or until the crust is golden brown.
Cool slightly, then top with fresh herbs, a bit of lemon zest, or an extra grind of black pepper.
Notes
Keep that butter cold - it will make a flaky crust. If it starts to melt, pop the dough in the fridge for 10 minutes. Chill the dough - a quick 30-minute chill helps the gluten relax and the butter firm up. Flake the salmon gently - Hot-smoked salmon is delicate, be gentle.