Wash the spinach in a bowl or in the sink. Cook it for 10 minutes in boiling water.
Meanwhile, mix the cream cheese, Gruyère, and crème fraîche together in a bowl. Cut the smoked salmon into strips and add it to the cheese mixture.
Once the spinach is cooked, drain it well, then sauté it for a few minutes over medium heat in a little olive oil to remove any excess moisture. Add it to the bowl and mix until you have a smooth paste.
Assembly and baking
Lay out the first sheet of puff pastry (leaving it on the parchment paper) on your work surface. Using a spatula, spread the filling over it, leaving a border around the edges.
Moisten the edge of the dough with water, then cover with the second sheet of puff pastry and seal the edge with your fingers. Brush with egg yolk so it turns a nice golden brown when baked! Sprinkle with poppy seeds.
Preheat the oven to 180°C. Place an upside-down glass in the center of the dough, and using it as a guide, divide the tart into 4 sections, then cut each quarter in half, and again in half, until you have 32 equal-sized wedges.
Gently twist the first slice, turning it two times to the right, being careful not to separate it from the center! The filling is quite thick, so it will be difficult to twist it any further. Continue this way with every other slice, then do the same with the remaining slices, but this time twisting them to the left.
Bake for 30 minutes and serve warm. The twists may stick together slightly, so use the tip of a knife to gently separate them before serving.