Cut the fresh rhubarb into 1-2 inch pieces.
Using a deep 10 inch skillet with an ovenproof handle, add the butter and sugar.
On medium heat melt the butter and sugar.
Arrange the rhubarb tightly on top, filling all gaps.
Cook over low heat for 20-30 minutes, or until it has caramelized (lightly browned) and excess liquid has evaporated.
Preheat the oven to 375ยบ F.
Roll out the pastry dough on a lightly floured surface, slightly larger than the skillet.
Cover the rhubarb mixture with the pastry dough.
Press down around the edge to enclose completely.
Bake for 25-30 minutes and the pastry is golden brown.
Remove from the oven and let rest for 5 minutes.
Cover the skillet with a serving plate and carefully flip over.
Serve Tarte Tatin with vanilla ice cream or whipping cream.