01 - Preheat the oven to 350°F.
02 - Line a 9-inch tart pan with the shortcrust pastry, trimming any excess. Prick the base with a fork and chill in the refrigerator for 10 minutes.
03 - Heat olive oil in a large skillet over medium heat. Add the chopped onion and sauté for 3 to 4 minutes until softened. Add minced garlic and cook for 1 minute.
04 - Add spinach to the skillet and cook until wilted and any moisture has evaporated. Remove from heat and allow to cool slightly.
05 - In a large mixing bowl, whisk together eggs, crème fraîche or heavy cream, nutmeg, salt, and black pepper until smooth. Fold in the cooled spinach mixture, crumbled feta, and half of the grated Gruyère cheese.
06 - Pour the filling evenly into the prepared pastry shell. Sprinkle the remaining Gruyère cheese on top.
07 - Bake the tart for 35 to 40 minutes or until the filling is set and the top is golden brown.
08 - Allow the tart to cool for 10 minutes before slicing and serving.