Have an instant-read thermometer, a strainer, and a blender at hand. Bring a few inches of water to a simmer in a large saucepan.
Put the sugar and lemon zest in a large heatproof bowl that can be set over the pan of simmering water. Off the heat, rub the sugar and lemon zest together between your fingers until the sugar is moist, grainy, and very aromatic. Whisk in the eggs, followed by the lemon juice.
Set the bowl over the saucepan, making sure that it does not touch the water, and start stirring with the whisk as soon as the mixture feels tepid to the touch. Cook the lemon cream until it reaches 180° F. As you whisk - you must whisk constantly to keep the eggs from scrambling - you'll see that the cream starts out light and foamy, then bubbles will get bigger, and then, as it gets closer to 180° F, it will start to thicken and the whisk will leave tracks. When the whisk leaves tracks, the cream is almost ready. Don't stop whisking or checking the temperature, and have patience - depending on how much heat you're giving the cream, getting to temperature can take as long as 10 minutes.
As soon as it reaches 180° F, remove the lemon cream from the heat and strain it into the container of the blender; discard the zest. Let the cream stand, stirring occasionally, until it cools to 140° F, about 10 minutes.
Turn the blender to high and, with the machine going, add the butter about 5 pieces at a time. Scrape down the sides of the container as needed as you incorporate the butter. Once the butter is in, keep the machine going - to get the perfect light, airy texture of lemon-cream dreams, you must continue to blend the cream for another 3 minutes.
Pour the cream into a container, press a piece of plastic wrap against the surface to create an airtight seal and refrigerate for at least 4 hours or overnight. (The cream will keep in the refrigerator for 4 days, or tightly sealed in the freezer for up to 2 months; thaw it overnight in the refrigerator.)
When your ready to assemble the tart, just whisk the cream to loosen it and spoon it into the tart shell. Top with stabilized whipped cream. Serve the tart, or refrigerate until needed. Yield: 1 9-inch French Lemon Cream Tart.