Heat the oven to 325°F (163°C). Line the bottom of a 9-inch springform pan with parchment paper. If using a water bath, wrap the outside of the pan securely in two layers of heavy-duty foil.

Stir together the almond flour, melted butter, low-carb sweetener, and salt. Press the mixture firmly and evenly into the bottom of the springform pan, then bake for 8–10 minutes and cool slightly.

Beat the softened cream cheese and sweetener on low to medium speed until smooth, scraping down the bowl as needed. Add the eggs one at a time, mixing only until each egg is incorporated.

Blend in the sour cream, vanilla extract, lemon juice, optional lemon zest, and salt. Avoid excessive beating because extra air can make the cheesecake puff and crack.

Pour the filling over the slightly cooled crust. Place the springform pan inside a larger roasting pan and add enough hot water to reach about halfway up the sides.

Bake for approximately 60–75 minutes, until the edges are set and the center remains slightly wobbly. Turn off the oven, crack the door, and leave the cheesecake inside for 30–60 minutes.

Remove the cheesecake from the water bath and let it cool to room temperature. Refrigerate for at least 6 hours, preferably overnight, before removing the sides of the pan and slicing.

For the sauce, simmer the raspberries, sweetener, lemon juice, and water over medium-low heat for 5–8 minutes, stirring until the berries break down. Strain for a seedless sauce if desired, and whisk in xanthan gum a very small amount at a time after removing the pan from the heat.

Cool the raspberry sauce completely. Serve each chilled slice with the sauce spooned over the top and add a few fresh raspberries if desired.