Stir in sugar, butter, nutmeg (optional), cinnamon and orange juice. Cook a bit longer, until texture is like jam.
Using the beater blade, combine ingredients on medium-high speed until well blended. Turn mixer off, add flour, and combine on low speed just until combined. Dough should be thick.
Fill each tart with a rounded teaspoon of filling (homemade or store-bought apricot jam). Bake for 10 minutes or until very lightly golden. Remove pan from oven and cool completely on wire racks.
When completely cooled sprinkle with powdered sugar, if desired. Store in an airtight container or freeze in freezer containers until ready to serve. Allow tarts to come to room temperature before serving for best texture.