Roll out the dough on a floured surface into a round disk, big enough to fit a non-stick 11?x1.5? with removable bottom tart pan (approximately 13 inches).
Roll dough over rolling pin and carefully unroll it onto the tart pan. Press it evenly with your fingers so the dough comes up the sides of the pan.
Prick the bottom and sides with a fork and bake for 12-15 minutes. Set aside to cool
In the meantime, sauté the onions in olive oil in a large skillet at medium-high heat for about 5 minutes. Add balsamic vinegar, agave and water. Reduce heat to low and continue cooking for another 5-7 minutes. Set aside
Beat eggs and egg whites in a large bowl. Season with ¼ teaspoon salt, add pumpkin puree and all the spices and mix well.
Whisk in greek yogurt until well incorporated. Add caramelized onions and mix well.
To prepare the walnut topping, combine walnuts, spices and brown sugar in a medium bowl and mix well so the walnuts are well coated with the spices
Pour pumpkin filling into pie crust, top with walnuts and bake at 350F for 45-50 minutes