Preheat the oven for 205°C (400°F)
Cut the plums in half and discard the core, then generously butter your tart pan. Set both aside.
In a small sauce pan, add the cream and sprigs of thyme. Bring to a simmer.
In a large skillet, add the sugar and bring to medium high heat to caramelize. Don't mix. Simply spread out the sugar by shaking the pan slightly and allowing the un-melted sugar granuales to fall into the melted sugar.
Let the sugar continue to melt and starts to color. You're looking for a medium amber color.
Strain the cream into a bowl and discard the thyme.
Once the caramel is a medium amber color, take it off the heat then pour the cream into the caramel, whisking non-stop.
Once the cream is combined with the caramel, add the cubed salted butter and continue to whisk until completely until combined.
Back on the heat and transfer your plum halves to the caramel, cut side down.
Cook for about 10 minutes, occasionally rotating the plums around in the pan to ensure that they cook evenly.
While the plums cook, roll out your puff pastry to a circle slightly larger than your tart pan and about 5mm thick (1/8 - 1/4inch) .
With a pair of thongs, transfer the plums to the tart pan, creating one layer of fruit. Pour the caramel evenly over the fruit.
Transfer the dough to the top of the plums and tuck in the edges in along the edges of the tart pan lip.
Cut 1-2 vents in the pastry dough for the steam to vent during baking.
Bake for 45 minutes or until the dough is golden brown and flaky.
Allow to cool for 5 minutes then place a serving dish on top of the pan. In one swoop, flip the pan over and let sit for a minute or so until the caramel and fruit releases.
Garnish with fresh sprigs of thyme then serve warm with homemade whipped cream or vanilla ice cream!