First, you will cook the beets. Heat oven to 200C and peel and slice the beets into thin rounds. Place them in an oiled baking dish and then cover with the cider vinegar and sugar, using a spatula to mix the beets around and coat them. Cook for 30 minutes.
Meanwhile, make the sauce by whisking together all of the ingredients. You could also use a food processor or hand blender for a super creamy sauce. Refrigerate until use.
Once the beets are out of the oven, reduce the temperature to 180C.
Roll out your puff pastry and press it into a flan or tart dish. If you only have a pie dish, that's OK too, but don't bring the pastry all the way up the side.
You are going to pre-bake the pastry a little. Poke holes at the bottom of the pastry and then bake for 10 minutes. (The holes will stop it from puffing up too much!)
Take out the part-baked crust. Arrange the beet slices on top and return to the oven.
Bake for 30 minutes, at which point the sides of the puff pastry should be slightly browned and all puffed up!
Drizzle the goats cheese sauce on top.
Finally, scatter some green onions or chives on top.
Serve!