Preheat the oven according to croissant package instructions.

In a mixing bowl, combine almond meal/almond flour, sugar, salt, butter, almond extract, eggs, lemon zest (if using), amaretto (or rum), and water. Mix until smooth to create the almond cream.

Cut each croissant in half lengthwise without cutting all the way through.

Fill each croissant with the almond cream mixture.

Place the filled croissants on a baking sheet.

Sprinkle sliced almonds on top of the filled croissants.

Bake in the preheated oven for about 15 minutes or until golden brown.

Dust with powdered sugar before serving.