Preheat the oven to 400'

Cut the plums into quarters and add them to a bowl with the orange liqueur and orange zest

Fit the Puff Pastry into a tart pan. (I didn't have a tart pan to use where I'm living right now, so I just used some aluminum foil to create a smaller form on a cookie sheet and then folded over the dough about 1/2 inch all the way around and used that for my edges.

Prick the dough to allow some steam to escape and bake the crust on the middle rack for 15-20 minutes until golden on the edges. Check every 5 minutes and prick the dough again if it starts to inflate.

Remove from the oven and increase the oven temp to 450'

In a small bowl, mix the mascarpone with the brown sugar and spread it on top of the crust.

Arrange the plums in rows to almost fill the top of the crust. Discard the leftover liquid.

Chop the walnuts and mix them with brown sugar and scatter them over the plums and the crust.

Bake on the top rack, just to toast the nuts and let the plums bond with the cheese; about 5 minutes.

Remove from the oven; let cool and then refrigerate until chilled; serve with vanilla ice cream or whipping cream.