Preheat the oven to 400°F. Line a baking sheet with parchment paper.
Unfold the thawed puff pastry sheet and use a rolling pin dusted with flour to roll the puff pastry out into a 10 × 17-inch rectangle.
Using a sharp knife or pastry cutter, divide the pastry sheet into eight equal-sized rectangles by cutting it in half both lengthwise and crosswise, and then dividing those quarters into halves. Score each of the rectangles with your knife to create a ½-inch border around the edge. In a small bowl, whisk together the plant-based milk and maple syrup. Using a pastry brush, generously brush the edges of the puff pastry with the maple syrup mixture. Place the puff pastry rectangles on the prepared baking sheet and bake until puffed, 16 to 18 minutes.
While the puff pastry is in the oven, make the filling. In a large bowl, combine the apples, butter, salt, cinnamon, red bean paste, and walnuts.
When the pastry rectangles have puffed, remove them from the oven. Tap the centers in so they deflate while the edges remain raised. Fill each pastry center with 3 to 4 tablespoons of the apple and red bean filling. Place the filled tartlets back in the oven and bake until the puff pastry is golden brown, an additional 7 minutes. Remove the tartlets and let them cool for 10 minutes before serving with ice cream.
The tartlets can be stored in the refrigerator for up to 3 days or in the freezer for up to 3 months.