Place the butter and sugar in the bowl of a stand mixer and combine with the paddle attachment until well incorporated. Add the egg and mix well, scraping the bowl. Add the salt, then flour, a half cup at a time, until combined. Don’t overmix.

Using your hands, gently gather the dough into a ball, and place it on a piece of plastic wrap. Press down to form a disc. Wrap the disc of dough with the plastic wrap and refrigerate for at least two hours.

When the dough is well chilled, remove the plastic wrap and place the dough on a large piece of parchment paper (about 14 inches) to roll out. If the dough is a bit sticky, flour both sides of the dough with the spelt flour. Roll out the dough to an 11-inch circle about a quarter inch thick.

Transfer the dough to the tart pan. I use the parchment paper to lift the dough and flip it over into the tart pan. Press the dough into the bottom of the pan and up the sides, making sure the dough is pressed into the edges where the sides meet the bottom. Trim the dough even with the top and return the pan to the refrigerator to chill for at least an hour.

Preheat the oven to 350 degrees Fahrenheit. Remove the tart pan from the refrigerator and prick the bottom of the crust a few times with a fork. Line the dough with a piece of parchment and fill with ceramic pie weights, rice, or dry beans.

Bake the crust for 15 minutes, then very carefully remove the parchment paper with the weights/beans/rice. Bake the crust for an additional 10 minutes, then remove it from the oven, and allow to cool.

Place all the ingredients, except butter, in a small saucepan. Heat mixture over medium high flame, and whisk until thickened, stirring constantly. Strain the mixture through a fine sieve into a bowl. Add the butter, one tablespoon at a time, and whisk to incorporate. Pour the mixture into the prepared crust. Refrigerate until set. Decorate as desired with Swiss meringue (see below) or thin lemon slices.

This meringue needs to be made in a double boiler. If you don’t have one, place the egg whites and sugar in a heatproof bowl which can sit on top of a saucepan, without falling in, or touching the water. I use my metal Kitchen Aid mixing bowl and set it above a smaller saucepan.

Place about an inch of water in the bottom pan and bring the water to a simmer. Place the bowl with the egg whites and sugar above and whisk until the sugar is dissolved. You can test it by quickly dipping your finger into the mixture and feeling no granules of sugar. The best way to do this is to use a thermometer and bring the mixture to 165 degrees.

Once the mixture is ready, transfer to your stand mixer, add the cream of tartar, andת using the whisk attachment, whisk on high for five to seven minutes until the mixture is white, fluffy, and holds stiff peaks. Add the vanilla while the mixture is whisking.

You can use a piping bag to decorate the tart with this meringue or spoon it decoratively on top.