Preheat oven to 350°F (175°C). Mix almond flour, 1/4 cup sweetener, and a pinch of salt. Stir in melted butter until crumbly, then press firmly into the bottom of a 9-inch springform pan. Bake 10 minutes until lightly golden, then cool slightly and reduce oven to 325°F (160°C).

Beat softened cream cheese until smooth, about 2 minutes. Mix in sour cream, 1 cup sweetener, vanilla, lemon zest, and salt until combined. Add eggs one at a time on low speed, mixing just until blended.

Wrap the springform pan with two layers of foil. Pour filling over crust and place in a roasting pan. Pour hot water into the roasting pan to reach 1 inch up the sides. Bake at 325°F (160°C) for 55 to 65 minutes, until edges are set and center jiggles slightly. Turn off oven, crack the door, and let cool inside for 1 hour.

Remove from water bath, cool completely on a rack, then cover and refrigerate at least 4 hours. Before serving, run a knife around the edge, release the springform, and slice into 12 pieces.