Preheat the oven to 350F and grease a 9-inch tube pan (aka angel food cake pan) well. Line the bottom with parchment (see recipe notes).
In a medium bowl, whisk together the walnuts, Swerve Brown, and cinnamon. Set aside.
In a large bowl, whisk together the almond flour, coconut flour, protein powder, baking powder, and salt.
In a large bowl, beat the butter with the Swerve until light and fluffy, about 2 minutes. Beat in the eggs one at a time, then beat in the vanilla extract, scraping down the beaters and the sides of the bowl as needed.
Add the flour mixture and beat until well combined. Then beat in 3/4 cup of the water. If the batter is quite thick, add additional water as necessary. The batter should be scoopable and spreadable but not quite pourable.
Spread about half the batter in the prepared pan, and then sprinkle with half of the walnut mixture. Spread the remaining batter in the pan and sprinkle with the remaining walnut mixture.
Bake 45 to 55 minutes, until golden brown and just firm to the touch. A tester inserted in the center should come out clean.
Remove and let cool in the pan, then run a sharp knife just around the edges to loosen and transfer to a plate.