Line the bottom of your slow cooker with parchment paper and lightly grease the sides.
Mix the almond flour, melted butter, and 3 tablespoons sugar-free sweetener in a bowl until crumbly, then press firmly into the bottom of the slow cooker to form the crust.
In a large mixing bowl, beat the softened cream cheese and 3/4 cup sugar-free sweetener with an electric mixer until smooth and fluffy, about 3 minutes.
Add the eggs one at a time, beating well after each addition, then mix in the lemon juice, vanilla extract, and sour cream until completely smooth.
Pour the cheesecake filling over the almond flour crust in the slow cooker and smooth the top with a spatula.
Place a kitchen towel under the slow cooker lid to catch condensation, then cook on low for 2.5 hours or until the edges are set but the center still jiggles slightly.
Turn off the slow cooker, remove the lid, and allow the cheesecake to cool completely in the slow cooker for 1 hour, then refrigerate for at least 4 hours or overnight before serving.