Preheat oven to 425 degrees F (220 degrees C). Place tart shells on a baking sheet; sprinkle raisins evenly into the bottom of each tart shell.
Whisk together butter, brown sugar, maple syrup, vanilla extract, and salt in a bowl; whisk in egg. Pour mixture into tart shells; sprinkle each tart with walnuts.
Bake in the preheated oven until pastry is light brown and filling is bubbly, about 12 minutes. Cool on a wire rack, about 10 minutes.
