1. Start by setting up your double boiler. Fill a saucepan with a few inches of water and bring it to a gentle boil. Choose a bowl that fits snugly on top of the pan without touching the water. This setup allows for gentle heating, which is crucial for our custard.
2. In your heat-proof bowl, combine 1 cup sugar and the finely grated zest of 3 lemons. Rub them together between your fingers for a minute or two. This process releases the oils from the zest, adding a beautiful fragrance and flavor to your mixture.
3. Next, whisk in 4 eggs and then the ¾ cup freshly squeezed lemon juice. Whisk until everything is well combined and smooth.
4. Place the bowl over the simmering water, making sure the bowl doesn’t touch the water. Keep whisking diligently. Monitor the mixture closely, and when it reaches a temperature of 180º F, you'll notice it starting to thicken. This is key, as you want a smooth custard, not scrambled eggs.
5. Once it thickens, remove the bowl from heat and strain the mixture into a blender. This step ensures that any bits of cooked egg are removed, leaving you with a silky custard.
6. Let the mixture cool for about 10 minutes before blending it. Gradually add 2 sticks plus 5 tablespoons butter, cutting it into tablespoon-sized pieces. Blend on high until all the butter is incorporated and the filling is creamy.
7. Pour the lemon filling into a container, pressing a piece of plastic wrap against the surface. This prevents a skin from forming; refrigerate it for at least 4 hours or overnight so it can set.
8. When you’re ready to assemble the tart, simply whisk the filling again to loosen it. Spoon it into your pre-baked 1 9-inch pie crust or tart shell and smooth it out with an offset spatula for an elegant finish.
9. For the best experience, serve it immediately! You can also chill it overnight if you want to make it ahead of time. The flavors deepen as it sits, making it even more delightful.
