Bake your crust: Preheat oven to 425°. Roll dough into a 12-inch circle. Fit into a 10-inch deep-dish tart pan with removable bottom; press into edges. Trim. Line piecrust with parchment paper, and fill with pie weights or dried beans. Place pan on a foil-lined baking sheet. Bake 12 minutes. Remove weights and foil, and bake 8 more minutes. Cool completely on baking sheet on a wire rack. Reduce oven temperature to 350°.
Deglaze pan with balsamic and add to onion mix. Remove from heat and drain any excess oil or fats from onions and pat with paper towel if needed.
Prepare your filling: Now that the crust is cooled, onions ready to go and oven is nice and warm, simply whisk the eggs, cream, salt, nutmeg and pepper together in a bowl. Place half the gruyere cheese on bottom of pie crust, then top with onions and gently pour your filling over top the onions and cheese, sprinkling the remaining cheese on top and finishing with parsley.
Bake at 350 degrees for 40 minutes until set.