Scoop out the filling of the pie slices and put them in a mixing bowl. Add in the milk and mix well. There's empty crust leftover, which you can discard or just pop in your mouth.
Preheat the oven to 350 degrees F, and boil some water. Spray 2 ramekins with cooking spray to prevent sticking. This preparation needs to happen first, because the next part is a little time sensitive.
Put the sugar in a saucepan over medium heat. This will begin the caramelization process as it melts down to syrupy consistency-about 10 minutes. Stir it frequently until it becomes brownish in color, and then immediately pour a layer of it in each ramekin before it starts solidifying as it cools. Then top each ramekin with the pumpkin mixture. Place the ramekins on a rimmed baking sheet, and then pour about a 1/2-inch layer of boiling water into the sheet to ensure even baking. Bake for 20 minutes.
After baking, let the ramekins cool, and then refrigerate them for a couple of hours until they're cold. Place a plate over each ramekin and then flip the two upside down so you can slowly lift the ramekin to reveal the flan inside. Garnish it with fresh mint if you have it and want it to look extra fancy.