Combine the sugar, maple syrup, and water in your saucepan fitted with a candy thermometer. Cook over medium heat without stirring, just swirling the pan occasionally, until it reaches 230°F. Keep a close eye on it here, because this step moves faster than you'd expect once it starts bubbling.
Pull the caramel off the heat right away and stir in the flaky salt. Pour it straight into your cake pan, tilting gently so it coats an even layer across the bottom. Let it sit at room temperature for about 30 minutes until it firms up slightly.
While the caramel cools, preheat your oven to 350°F. This gives the oven plenty of time to reach a stable temperature before the custard goes in. An even oven temperature really does make a difference in how smoothly this flan bakes.
In a stand mixer or large bowl, blend the condensed milk, evaporated milk, pumpkin puree, and mascarpone until completely smooth. Whisk in the eggs, vanilla, cinnamon, allspice, and orange zest. Take your time here so there are no streaks of mascarpone left behind.
Set your caramel-lined pan inside the roasting pan. Pour the custard slowly over the back of a spoon so it doesn't disturb the caramel layer underneath. This little trick keeps the caramel from mixing unevenly into the custard.
Carefully add hot water to the roasting pan until it reaches halfway up the sides of the cake pan. Bake for 70 to 75 minutes, until the edges are set but the center still has a gentle jiggle. The water bath is what keeps this custard so smooth instead of rubbery.
Lift the pan out of the water bath and set it on a wire rack to cool completely. Once it's at room temperature, transfer it to the fridge and chill for at least 3 to 4 hours, though overnight works even better. This resting time is non-negotiable for clean slices.
Run a sharp knife around the edge of the flan to loosen it. Place a serving platter upside down over the pan, then flip both together in one confident motion. Give it a gentle shake, and watch that maple caramel cascade down the sides.