Preheat oven to 350F (180C).

To make the crust, combine graham cracker crumbs, brown sugar, and melted butter and mix until it has the texture of a moist sand. It should clump together when you squeeze some in your hand.

Press into a pie pan (I used a 9 inch standard glass one) and press the mixture up the sides. Feel free to use the bottom of a glass to flatten the base of the pie crust.

Bake the crust for 8-10 minutes, you'll see the edges of the crust brown. Remove from oven and let cool while you prepare the filling.

First zest your key limes into your pot (zesting over the pot means you keep the fragrant oils too!) You should get about 1 tbsp of key lime zest. You can also set aside about 1 tsp of zest to garnish if you like.

Juice your key limes into the pot, you should get at minimum 1 cup of key lime juice. A little extra is fine, anywhere between 1 cup to 1 cup + 2 tbsp juice! Add 6 tbsp cornstarch and whisk well to incorporate. Turn on the heat and whisk until thickened, it should be the consistency of a very thick gravy.

Turn off the heat, and quickly whisk in the sweetened condensed oat milk. It will foam up slightly, but just whisk until incorporated, then pour immediately into the cooled graham cracker crust.

Allow to cool (about 30 minutes), then transfer to the fridge to set the filling (at least 1 hr).

In a large mixing bowl suitable for an electric beater (you could do this by hand if you have the stamina), combine vegan whipping cream, icing sugar, and cream of tartar.

Beak/Whisk with an electric mixer until you get stiff peaks (about 5-7 minutes).

Add on top of the chilled key lime filling, and then garnish with key lime zest if desired.

Optional: refrigerate to allow the whipped cream to set, but you can serve immediately after topping if desired :)