After baking the tart shell, keep the oven at 375° F.
In a medium bowl, whisk together the egg yolks and eggs until combined. Add sugar and whisk again. Add the lemon juice, lemon zest and salt, then whisk once more (no over-whisking in these steps–just about 10 seconds per addition).
Transfer mixture to a nonreactive saucepan (stainless steel or enamel, not aluminum or copper). Add the butter pieces and cook over medium-low heat, stirring constantly. Lemon curd is finished when it reaches a thin, sauce-like consistency, about 5 minutes.
Immediately pour the lemon curd through a fine mesh sieve into a nonreactive bowl. Stir in the heavy cream, then transfer the mixture to the warm prebaked tart shell (see notes).
Place the filled tart shell on a baking sheet and bake it for 10 to 15 minutes. The tart is done when the edges are set and the center jiggles slightly when gently shaken. The filling will be shiny and opaque.
Allow the tart to cool to room temperature, then chill the tart in the refrigerator for 1 hour. Remove the rim from the tart pan, garnish the tart, slice and serve.
Notes
Warm tart shell: If the tart shell cools before starting the curd, simply place it in the oven for 5 minutes and heat it until warm. You can keep a piece of aluminum foil over it to keep it warm while sitting on the counter.Store lemon curd tart in an airtight container or tightly covered in the pan. It will keep in the refrigerator for up to 2 days.Freeze lemon curd tart for up to 3 months. Once chilled, freeze the tart for 2 hours until firm, then wrap it in plastic wrap and aluminum foil. Freeze on a flat surface. When ready to serve, defrost in the refrigerator overnight.Recommended tools: Tart pan, nonreactive saucepan, fine mesh sieve