Preheat oven to 350°. Pulse 3/4 cup all-purpose flour, almonds, and a pinch of sea salt in a food processor 3 times or until combined. Beat powdered sugar and 1/4 cup softened butter at medium speed with an electric mixer 5 minutes or until pale and fluffy. Gradually add flour mixture and 1 egg yolk, beating at low speed until a dough forms. Press dough into a lightly greased 13- x 4-inch tart pan with removable bottom. Chill 10 minutes. Bake 14 minutes or until golden. Cool completely. Whisk 6 egg yolks until thick and pale. Whisk together sugar, 6 Tbsp. all-purpose flour, and a pinch of sea salt. Combine milk and lemon zest in a saucepan; cook over medium heat, stirring constantly, 5 minutes or just until it begins to steam. Whisk in sugar mixture, whisking constantly just until bubbles appear. Gradually stir one-fourth of hot milk mixture into yolks; add to remaining hot milk mixture, stirring constantly. Bring mixture to a low boil over medium heat; remove from heat. Stir in 2 Tbsp. butter and amaretto liqueur. Transfer to a bowl; place plastic wrap directly onto custard. Cool 1 hour. Spread into crust; top with thinly sliced plums. Brush with warm apricot preserves.