Heat the lemon juice and zest in a small saucepan over medium-high heat until just boiling. Remove from the heat.
Whisk the eggs, sugar and salt together in a medium saucepan until well combined. Slowly drizzle the eggs into the lemon mixture, whisking constantly.
Cook the egg mixture over medium heat, stirring constantly with a rubber spatula, until the custard is thick, about 8 minutes. Remove the custard from the heat and stir in the butter. Once all of the butter melts and is fully incorporated, strain the custard through a fine mesh sieve into the tart shell.
Leave the tart to set up in the refrigerator for at least 1 hour 30 minutes.
Serve with a dollop of whipped creme fraiche and whipped cream folded in together.