Prepare and rest your pastry shell. Line your tart pan with pastry and rest it in the freezer for 30 minutes. A cold shell going into a hot oven holds its shape better and shrinks less.
Blind bake the pie crust. Line the shell with parchment paper, fill with pie weights or dried beans, and blind bake at 350F (180 °C) until lightly golden brown. Remove the pie weights and parchment. Place an empty baking sheet on a lower oven shelf to preheat while you prepare the filling.
Prepare the salmon. Bake a fresh salmon fillet at 350F (180 °C) for 12 to 15 minutes until just cooked through, then cool and flake. Or use leftover cooked salmon, canned salmon drained and flaked, or smoked salmon torn into pieces.
Prepare the spinach: If using fresh, then wilt the fresh spinach in a dry pan over medium heat for one to two minutes, using only the water left on the leaves from washing. Do not add extra liquid and do not overcook. You want it just wilted. If using frozen spinach, then thaw it completely.
Rinse the spinach. Immediately rinse under cold running water. This stops the cooking and keeps the color a bright, vivid green.
Squeeze the water out of the spinach. Really squeeze, with your hands. Think of the spinach like a wet sponge and wring it out until it is almost completely dry. This is the most important step in the entire recipe. Any water left in the spinach will seep into your custard and prevent it from setting.
Make the custard. Whisk the eggs and crème fraîche or cream together until smooth. Add your herbs, nutmeg, salt, and black pepper and whisk briefly to combine. Pour into a jug if you have one. It makes it much easier to pour evenly into the shell without disturbing the filling.
Assemble the salmon spinach quiche. Pour a thin layer of custard into the base of the blind-baked shell. Scatter the cooled spinach evenly on top. Keeping it at the bottom means it stays tucked under the custard and salmon during baking and does not rise or dry out. Arrange the flaked salmon over the spinach.
Pour the remaining custard slowly over everything and let it settle on its own.
Bake the quiche. Place the quiche on the preheated baking sheet at 180C (160C fan / 350F) and bake for 30 to 35 minutes. The edges should be set and the centre should still have a gentle wobble, like a just-set creme brulee. It will finish setting as it cools. If the centre is completely firm, it has overcooked.