Prepare the dough.
In a food processor, add the flour and half of the cold butter. Blend for 20 seconds, intermittently. Then add the rest of the butter and blend for another 5 seconds.
Add cold water with lemon juice and knead for 5-10 seconds until a dough ball is formed. Do not over-process.
Put the dough on a surface sprinkled with flour, spread it with a rolling pin, then wrap it back together like you would close an envelope from all sides; cover it with a plastic wrap and put it in the refrigerator for at least an hour or two.
Make the galette.
When the dough is ready, take it out of the refrigerator, stretch it until you get a circle and put it on a parchment paper-covered baking tray.
Heat the oven to 200 C (390F).
Spread cream cheese on the dough, sprinkle with onion powder, then arrange the round slices of beet, so that they cover the entire surface of the dough, except for the edges.
Roll the edges over the beetroot filling.
Drizzle with olive oil, sprinkle with thyme and oregano and put in the oven for 30-40 minutes, or until the edges are browned.
Serve sprinkled with freshly grated Parmesan.