Oooh, that's is such a good idea I'm nicking it off you. If you're asking how, I'll buy the pasta pastry (cannot make it myself) either ready shaped or cut it out. The filling I'll make using oil, flour (to thicken) and horseradish - may add some mushrooms too, see how it all looks - and probably pepper and a little cream. Let me know how you get on, and I'll do the same. (Not tomorrow though, as I'm doing marinaded chicken garam masala thingies)
I'd mix mascarpone, white breadcrumbs and an egg yolk in with the horseradish. Probably use the creamed ready made horseradish rather than the grate it yourself variety, just to give it a bit more moisture.