- Preheat oven to 400 degrees and line a 9-inch tart pan with removable bottom with your favorite pie crust recipe or refrigerated dough.
- Pierce tart crust several times with a fork. Crumple a sheet of parchment paper. Open it up, place over crust and fill with pie weights, dried beans, or uncooked rice. Bake for 10-12 minutes or until golden. If using a store-bought frozen pie crust, follow package directions. Set aside to cool and reduce oven temperature to 325 degrees.
- Heat oil and butter in a medium sauté pan until foamy. Add leeks and a pinch of salt and gently sauté for 7 minutes or until tender. Stir in mustard and let cool.
- Whisk eggs and cream in a small bowl. Sprinkle cheese over the tart and scatter leeks on top. Pour egg mixture into tart shell and garnish with fresh thyme leave. Bake until filling is set, 35 to 40 minutes. Remove from oven and let stand for 5 minutes before slicing.