Step 1
Preheat oven to 350 degrees F (175 degrees C).
Step 2
Whisk all-purpose flour whole wheat flour almond meal 1 tablespoon sugar lemon zest and salt together in a large bowl. Cut in butter with a pastry blender until mixture is in pea-sized crumbles. Stir in buttermilk until dough comes together.
Step 3
Press dough evenly over the bottom and 1/2 inch up the sides of a 12x16-inch tart pan or sheet pan.
Step 4
Bake in the preheated oven until lightly browned about 20 minutes. Place pan on a wire rack to cool completely about 1 hour.
Step 5
Whisk egg yolks 2/3 cup sugar and cornstarch together in a heavy-bottomed pot. Whisk in milk. Add whole basil sprigs.
Step 6
Cook milk mixture over medium heat whisking constantly until mixture begins to steam about 8 minutes. Reduce heat to low. Remove basil sprigs with tongs and discard. Whisk vigorously until custard thickens about 1 minute.
Step 7
Set a wire mesh strainer over a large bowl. Pour custard into the strainer; use a rubber spatula to press it through the strainer into the bowl. Whisk vanilla extract into the custard. Press a piece of plastic wrap directly onto the surface. Refrigerate until cold about 3 hours.
Step 8
Pour cooled custard over crust; spread evenly to edges of crust with a rubber spatula. Arrange apricot slices on top.
Step 9
Scoop apricot jam into a small microwave-safe bowl; microwave on high until melted about 1 minute. Brush jam over apricot slices.