Prepare the pie crust according to package instructions. Flatten into a disk and chill in the refrigerator while preparing the filling.
In a large saute pan over medium heat, heat the canola oil. Add the onions and season with salt and pepper. Cook, stirring occasionally, until the onions are dark golden brown, 10 to 15 minutes. Remove from the heat.
In the meantime, using the slicing blade attachment of a food processor, or with a box grater, slice half the potato into 1/8-inch-thick slices. Place in cold water to avoid browning. (Reserve the remaining half of potato for another use, such as Round 2 Recipe Open Faced Roast Pork and Onion Sandwich.)
In a small bowl, stir together the cream cheese, Italian seasoning, mustard and season with salt and pepper.
Remove the pie crust from the fridge and roll into a 12-inch round about 1/4-inch thick. Place on a parchment-lined baking sheet. Spread the cream cheese mixture in an even layer over the pie crust, making sure to leave a 1-inch border around the edge.
Dry the potatoes thoroughly and place an even layer on top of the cream cheese, overlapping slightly to fit and keeping 1/2-inch from the edge of the cream cheese. Over the top of the potatoes place an even layer of caramelized onions. (Reserve 1/2 cup of the onions for another use, such as Round 2 Recipe Open Faced Roast Pork and Onion Sandwich.) Fold over the edges of the crust to create a 1-inch-wide border around the tart filling.
Bake until the crust is golden and the potatoes are cooked through, 30 to 35 minutes. Allow to cool for 5 minutes before slicing.