You need:

For the pastry

230g (1¾ cups) plain (all-purpose)

flour

125g (½ cup plus 1 tablespoon)

cold unsalted butter, 

fine salt

1 egg yolk

2 tablespoons milk

For egg wash - 1 egg yolk and 1 tbs water

1 tsp black sesame seeds to sprinkle the sides of the dough

For the filling

3 medium sized beetroots (preferably different colors)

3 tbs extra vergin olive oil 

1tbs white wine vinegar (for yellow and orange beets)

1 tbs red wine vinegar (for the red beets)

1tsp fresh theme leaves

1 tbs liquid honey

How to: 

Start with baking your beetroot. Wrap each beetroot in a foil and bake 50-60 min depending on the size of your beetroot. Meanwhile prepare your dough and brown the onions. 

Peal and cut the red onions in half. Brown them until soft and tender cut side down with a little olive oil. Set aside to cool when done. 

In the bowl of a freestanding mixer, place the flour, butter and salt.

Attach the paddle and mix on a medium speed until the butter has been incorporated into the flour and resembles fine breadcrumbs. Add the egg yolk and milk and continue to mix, this should only take arround 60 seconds, depending on your mixer. Turn out the pastry onto a work surface.

Flatten the pastry with the palms on a piece of baking paper. Roll out to an approximate depth of 5mm, trying to keep it in a circular shape. 

When the beetroots are all baked and cooled, peal the skins off, and cut them in half and then into thin  slices. Coat the red beetroot with 1 tbs red wine vinegar and 1 tbs of olive oil. Do the same with white wine vinegar, olive oil and yellow/orange beetroot. 

When the beets are coated and glossy arrange them nicely on a pastry sheet, add the onions and drizzle with the liquid honey on top. Flap the sides of the pastry to make a round galette. Add the egg wash on the sides and finally sprinkle the sides with black sesame seeds. 

Bake for 45 min 190C. Add the fresh theme leaves when ready. 

Eat it hot or cold with a simple salad. 

Enjoy