Add eggs one at a time and mix at slow speed until dough pulls itself from sides of bowl.
Turn dough onto clean surface and knead by hand until smooth.
Fit a medium-sized, heat-proof mixing bowl over a similar sized pot to make a double boiler. Make sure bowl fits properly with several inches between bowl and bottom of pot.
Remove bowl and bring about an inch of water to a simmer in pot. Meanwile, off the heat, in the same mixing bowl, whisk the whole eggs, egg yolks, sugar, lemon juice, and lemon zest until well combined.
Place bowl over the double boiler, making sure bowl does not touch water. Cook the lemon curd until it thickens, stirring every few minutes to prevent the curd from burning on the sides. Add in the butter and allow to dissolve. If necessary pass through a fine strainer.
Remove from heat and chill. Set aside until ready to fill tart shells.
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