Make Almond Crust
Preheat oven to 375. Grease a 9" round or 4x15 rectangular tart pan with removeable bottom.
In food processor, process almonds until finely ground. Add flour and powdered sugar and pulse until combined. Cut chilled butter into 8 pieces and scatter over almond mixture. Pulse until just combined. Drizzle cold water over and pulse mixture just until it starts to come together.
Place dough into prepared tart pan and press evenly over the bottom and 3/4-1 inch up sides of pan. Prick all over with a fork to keep dough from bubbling up and bake in preheated oven 12-14 minutes, until lightly golden on edges.
While Crust Bakes Make The Filling
Zest lemons and set zest aside. Squeeze lemons to get 2/3 cup lemon juice.
In a medium saucepan combine lemon juice, lemon zest, egg yolks, whole eggs, sugar and pinch of salt. Whisk until combined and no traces of egg whites remain.
Place lemon/egg/sugar mixture over medium-low heat, add butter and whisk constantly until mixture begins to thicken and reaches 170 degrees f on candy thermometer. If you don't have a thermometer watch for the mixture to begin bubbling and turn off the heat before it actively boils. It should take 5-8 minutes.
Pour thickened lemon mixture into a fine mesh sieve/strainer set over a heat proof bowl. Push mixture through strainer using a rubber spatula until only the solids (this will mostly be zest) are left. This will leave you with a perfectly smooth lemon filling in the bowl.
Stir heavy cream into strained lemon filling until smooth. Pour this into the baked tart shell and return to the 375 oven for 13-15 minutes. The tart is done when the edges are set but the center still jiggles slightly when you shake the pan.
Cool on a wire rack until it's room temperature and then chill for at least 2 hours.
Optional Whipped Cream
Place cold whipping cream into a chilled bowl. Beat with whisk attachment on high until cream thickens enough that whisk leaves a trail. Add powdered sugar and either vanilla or Limoncello. Continue beating until thick enough to hold a shape when piped. Pipe around edges of chilled tart or just serve a dollop on each piece.
