Preheat oven to 375°F (190°C).
Place tart shell in a tart pan and prick bottom with a fork.
Arrange apricot halves cut-side up inside the tart shell.
In a bowl, mix sugar, lemon juice, melted butter, and vanilla extract.
Brush mixture evenly over apricots.
Warm apricot jam slightly and spread lightly over fruit.
Sprinkle sliced almonds if using.
Bake for 35–40 minutes until crust is golden and fruit is tender.
Let tart cool slightly before removing from pan.
Dust with powdered sugar before serving if desired.