In a medium saucepan, combine sweetener for the custard, xanthan gum, salt, and zest.

Place saucepan on the stove and whisk in about a tablespoon of the heavy cream. Turn heat to low and gradually add the remaining cream, 1-2 tablespoons at a time. Be sure to whisk thoroughly between each addition.

Once all of the cream has been added, whisk in the egg yolks. Increase heat to medium-low, stirring frequently.

When the mixture begins to simmer, remove from heat and stir in vanilla and lemon extract.

Transfer the mixture to 4-oz. ramekins or your desired oven-safe serving dish. Prepare the meringue right away, so the custard will still be warm when the meringue is spread on top.

In a small bowl, whip together the egg whites and cream of tartar using an electric mixer at medium-high speed until soft peaks form.

Add sweetener, and vanilla, then beat at high speed until stiff peaks form.

Evenly spoon the mixture onto each of the custards. Be sure to seal the edges of the custard with the meringue. To create peaks, press down on the meringue with a spoon and then briskly lift upward.

After spreading the meringue, adjust top rack of your oven so it is about 8 inches below the broiler element. Turn the broiler to high heat. Transfer the ramekins to a cookie sheet and place the sheet on the top rack. When the meringue begins to brown (about 10-20 seconds), turn the cookie sheet around and broil for another 10-20 seconds. The tips of the meringue should be brown, but the base should still be white. Do not over-brown the meringue as over-browning can make the meringue tough.

Once the meringue has been toasted, transfer the ramekins to the refrigerator until serving.