Preheat the oven: to 350°F/ 175°C.
Make the crust: In a food processor or blender, combine the pretzel chips, butter, and sugar and process until the pretzels have broken down into fine crumbs and the mixture looks like moist sand. It should easily hold together when squeezed.
Transfer the mixture to an 8-inch tart pan. Push the mixture outward to create an even ring around the perimeter, then use a measuring cup to press the mixture firmly against the sides, then using the bottom of the measuring cup, flatten the crumbs into an even layer with no bare spots.
Bake the crust: Until it’s fragrant, firm to the touch, and the outer edges are golden, 12-15 minutes. Set the pan aside.
Make the filling: In a stand mixer or a large bowl if using a hand mixer or whisk, combine the cream cheese and sugar, and beat on medium-high until completely smooth.
Add the egg and beat until incorporated. Add the pumpkin puree, sour cream, vanilla, spices, orange zest, and salt and mix until combined. Increase the speed to medium-high and beat, scraping down the sides of the bowl occasionally, until the filling is very light and smooth, about 3 minutes.
Pour the batter into the cooled crust and smooth the top. Lightly tap the pan on the counter to pop any air bubbles.
Make the tahini swirl: In a small bowl, add the tahini and stir in water, 1 teaspoon at a time, until it reaches a thin consistency. Add in a pinch of salt.
Drizzle tahini mixture over the cheesecake filling and swirl gently with a knife.
Bake at 325°F / 160°C until the filling is set around the edges and slightly wobbly in the center, around 20-25 minutes.
Turn off the oven and leave the door slightly open using a wooden spoon to hold it in place. Let the tart gradually cool for about 30 minutes. Remove from the oven and allow it to cool at room temperature for another 30 minutes before transferring to the fridge. This gradual temperature change helps prevent cracks to form. Chill at least 2 hours or overnight.
Make the marshmallow: In a heatproof bowl over a simmering pot of water, whisk the egg whites and sugar until the sugar has fully dissolved (about 160°F / 70°C). If you don’t have a thermometer, the mixture is ready when it feels smooth and warm to the touch and the sugar has completely dissolved. Rub a small amount between your fingers, if it feels smooth and not gritty, it’s ready.
Remove from heat and immediately whip with a mixer or using a whisk until stiff, glossy peaks form and the bowl has cooled.
Spread or pipe on the tart. Using a kitchen torch, carefully toast the marshmallow topping until golden, or place under the broiler for about 1 minute, watching closely so it doesn’t burn. Serve.