French Savoury Tart

Name: French Savoury Tart
Cuisine: European, Mediterranean, French
Ingredients: 1 ¼ cups all-purpose flour (for the crust); 7 tablespoons cold butter, cubed; 1 large egg (for the crust); 1 tablespoon cold water (as needed); 4 slices bacon or lardons, diced; 1 small onion, finely chopped (optional); 3 large eggs (for the filling); 1 cup heavy cream; ½ cup whole milk; ¾ cup Gruyère cheese, grated (optional); Salt and freshly ground black pepper; Pinch of ground nutmeg (optional)