Arrange an oven rack in the center position. Preheat the oven to 350°F (175°C)

Place the empty 9-inch round cake pan inside the larger roasting pan. Set both pans on a stable, heat-resistant surface near the stove.

Combine the granulated sugar and ¼ cup water in a medium heavy-bottomed saucepan. Stir gently only until all the sugar is moistened.

Place the saucepan over medium heat. Cook without stirring for 8 to 12 minutes. If the color is developing unevenly, carefully swirl the saucepan by its handle.

Watch the caramel closely once it begins turning golden. Remove it from the heat when it reaches a medium amber color and smells lightly toasted. Do not let it become very dark or smoky, or it may taste bitter.

Carefully pour the hot caramel into the 9-inch cake pan. Using oven mitts, immediately tilt and rotate the pan so the caramel covers the bottom evenly. Set it aside for approximately 10 minutes while preparing the custard. The caramel will harden as it cools.

Add the pumpkin purée, sweetened condensed milk, evaporated milk, eggs, vanilla, cinnamon, nutmeg, ginger, and salt to a blender.

Blend on low speed for 20 to 30 seconds, stopping as soon as the mixture is smooth. Avoid prolonged high-speed blending, which can incorporate too much air.

Allow the mixture to rest for 2 minutes so large bubbles can rise. Pour it through a fine-mesh strainer into the caramel-coated pan.

Cover the cake pan tightly with aluminum foil. The foil helps prevent excess browning and keeps water droplets away from the custard.

Carefully place the roasting pan on the center oven rack.

Pour 4 to 6 cups of hot water into the roasting pan around the covered cake pan. Use enough water to reach approximately halfway up the outside of the cake pan. Do not pour water into the custard.

Bake at 350°F (175°C) for 55 minutes. Carefully lift one edge of the foil and check the center.

Continue baking for another 5 to 10 minutes, or until the outside is set and the center moves with a gentle, unified wobble when the pan is nudged. It should not look liquid or ripple like uncooked batter. Total baking time should be approximately 60 to 65 minutes.

If checking with an instant-read thermometer, the center should reach approximately 175°F (79°C). Avoid touching the bottom of the pan with the thermometer.

Carefully remove the cake pan from the water bath using oven mitts. Place it on a wire rack and remove the foil.

Let the flan cool at room temperature for 1 hour. Do not leave it unrefrigerated for longer than 2 hours.

Cover the cooled flan and refrigerate it for at least 4 hours. For the firmest texture and easiest unmolding, chill it overnight.

Run a thin knife around the outside edge of the chilled flan. Do not cut deeply into the custard.

Place a large rimmed serving plate upside down over the cake pan. Hold the plate and pan firmly together, then invert them in one confident motion.

Leave the pan in place for 20 to 30 seconds so the flan and caramel release. Slowly lift off the pan. Spoon any caramel remaining in the pan over the flan.

Cut the flan into 10 slices and serve cold.