Press the puff pastry into the bottom of your pan and up the sides. Do not worry about using a fancy pan, the puff pastry will hold its shape regardless of the pan you use.
Bake according to package directions.
When slightly cooled, gently press the pastry down so that it creates a shell for your filling. Cool completely.
Beat the mascarpone cheese, powdered sugar, vanilla paste, and salt together until fluffy (about 5 minutes).
Spread the cheese filling into the cooled puff pastry and top with the cooled rhubarb. Delicious!