What your eyes see definitely doesn't match what your mouth tastes… It looks like pumpkin pie, it smells like pumpkin pie, but it's not pumpkin pie. Confused? Go ahead, give it a try. You'll be surprised how good contradiction tastes.
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Combine all of the ingredients in a medium-sized bowl and mix well so the walnuts are well coated with the spices.
Preheat the oven to 350 degrees.
Lightly flour a work surface and roll out the dough into a round disk large enough to fit a nonstick 11-by-1 ½-inch tart pan with removable bottom (about 13 inches).
Roll the 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.
Meanwhile, sauté the onions in olive oil in a large skillet over medium-high heat for about 5 minutes. Add the balsamic vinegar, agave nectar, and water. Reduce the heat to low and continue cooking for 5-7 more minutes. Set aside.
Beat the eggs and egg whites in a large bowl. Season with the salt, add the pumpkin purée and all of the spices and mix well. Whisk in the Greek yogurt until well incorporated. Add the onions and mix well. Pour the pumpkin filling into the pie crust, top with the walnuts, and bake for 45-50 minutes.