In a medium-sized saucepan, whisk together the whole eggs, eggs yolks, sugar, lemon zest, and salt until light and creamy. Slowly whisk in the lemon juice.
Turn the burner to a medium-low heat setting, and whisk the lemon filling until it reaches 165° F. This will take between 5-8 minutes. Once the curd starts to bubble and thicken, coating the back of a spoon, remove from the heat.
Place a stainless steel fine mesh sieve over a large bowl with the cold butter and carefully pour the lemon filling into the sieve to strain any cooked eggs and zest. Use a spatula to help press the filling through the sieve, tapping the sieve every once in a while to push through any excess filling. Occasionally wash out the bottom of the sieve discarding any zest that clogs the mesh, and continue the process until all the filling is strained. You should have about 2 cups worth of filling.
Whisk together the lemon custard and butter until nice and smooth, then pour the filling into a par-baked tart crust. Depending on the type of crust you make, it may be beneficial to add a pie shield to avoid the top of the crust from burning.
Place the tart on a baking sheet to allow for easy retrieval, and bake the tart at 350° F for 12-15 minutes. The tart should be slightly wobbly but not puffy or cracked.
Remove from the oven and allow the tart to sit at room temperature for 1 hour. Then place the tart in the refrigerator for at least 4 hours to chill. For best results, chill overnight.
Keep the tart refrigerated until ready to serve. Prior to serving, remove the shell from the tart pan and top with freshly made whipped cream, fresh berries, or lightly dust with powdered sugar.