Preheat oven to 350°F. Line a 7-inch1 springform pan or cake pan with parchment paper. Set aside a large oven-safe pan for the water bath.
Make the crust: Mix together oat flour, almond flour, brown sugar, cinnamon, and salt. Break up any large clumps of sugar. Add walnut butter and stir to coat everything, then use your fingers to rub the walnut butter into the dry ingredients. Add water 1 tbsp at a time until the mixture just holds together when you press it between your fingers.
Par-bake crust: Press the crust into the lined pan. Place on the middle rack of your oven and bake for about 15-20 minutes until firm and the edges start to lightly brown. Cool for 5-10 minutes before adding the filling.
Reduce the oven heat: Reduce oven temperature to 325°F.
Make the filling: Drain and rinse the soaked cashews, then add to a blender along with the rest of the filling ingredients. Blend until very smooth, then pour on top of the crust and smooth out the top. Tip: If your blender is not powerful enough and your filling still looks a little grainy, strain it through a sieve before adding to the pan.
Boil water: Boil about 4 cups of water for the water bath.
Bake: Place the cheesecake on the middle rack of the oven. Immediately place the oven-safe pan for the water bath on the bottom rack of the oven, and pour the boiling water into the pan so it's about 1-inch deep. Quickly close the oven door and bake for 45-60 minutes until the center is just set but still has a very slight wobble.
Cool: Turn off the oven and let the cheesecake cool for 1 hour with the door slightly ajar. Remove the cheesecake and let it cool completely to room temperature, then cover and chill in the fridge overnight.
Notes
1) Pan Sizes: You can use an 8-inch pan but expect a shorter cheesecake, and start checking the cheesecake 5-10 minutes earlier. You can also try a 6-inch pan if it's deep enough to hold the filling. It will probably need an extra 5-10 minutes of baking.