Golden beet tarte tatin

Pretty delicious, for just being essentially beets and puff pastry. The original recipe called for baby beets, halved, but I cut and peeled three big golden beets instead. Next time I might try a combination of beet colors.

1 sheet puff pastry, thawed
1 tablespoon butter
1 tablespoon canola 
or olive oil
2 teaspoons cider vinegar
2 teaspoons brown sugar
Sea salt and freshly ground black pepper
3 big golden beets, peeled and cut into thin chunks

Melt the butter with the oil in a cast-iron skillet. Add the cider vinegar, sugar, and some salt and pepper, stir well, then 
add the beets and toss to coat. You want the beets to fill the pan snugly, in a single layer. Cover the pan with foil, transfer to the oven, and roast for 30 to 40 minutes at 375, until the beets are tender.

Take the pan from the oven and rearrange the beet halves neatly, placing them cut side up. Lay the pastry sheet over the beets, patting it down and tucking in the edges down the side of the pan. Return to the oven and bake for 20 minutes, until the pastry is fully puffed up and golden brown.

Leave the tarte to cool in its pan for about 15 minutes, then turn it out by putting a plate over the top and inverting it. Pour any juices left in the pan back over the beets.