Preheat oven to 425 degrees Fahrenheit/220 degrees Celsius.
Melt vegan butter in a saucepan. Add the flour and saute for a couple of minutes, stirring constantly. You'll know when the roux is done when it smells aromatic and loosens up. It will be lightly blonde in color.
Stir in the milk slowly, mixing or whisking as you go, to keep lumps from forming. Once you have a smooth sauce, stir in nutmeg, pepper and red pepper flakes, if using. Mix. Cook the sauce a couple more minutes until it begins to thicken and coats the back of a spoon.
Add salt to taste and turn off the heat. Reserve all 4 oz vegan mozzarella shreds for layering over the bechamel sauce.
Roll out the puff pastry sheet on a lightly floured surface. It should be approximately 14 inches long and 9 inches wide. Transfer the puff pastry carefully to a baking sheet.
With a knife, carefully score a one-inch edge along the border of the puff pastry, taking care not to cut all the way to the bottom. Spread the bechamel sauce evenly inside the scored border, then layer all 4 oz vegan mozzarella shreds over the sauce.
Layer the trimmed asparagus spears over the cheese. Brush lightly with olive oil and sprinkle on some salt and pepper.
Bake for 20-25 minutes or until the puff pastry is golden brown and puffed. Cool on a rack for five minutes, then slice and enjoy!