Cut butter into cubes. Quickly work flour, 1/2 teaspoon salt and butter into a crumbly dough with your fingers. Add 60 ml cold water and knead quickly to a smooth dough. Form dough into a ball, wrap in foil and chill for about 30 minutes.
In the meantime peel the onion and cut it into fine strips. Wash, clean, peel and roughly grate the beetroot. Wash thyme, shake dry and chop finely, except for 2 stems for garnishing. Heat oil in a pan.
Fry the onion for about 4 minutes over a low heat, turning it over. Add beetroot, thyme, sugar, vinegar and 50 ml water. Simmer over medium heat for about 10 minutes. Season to taste with salt and pepper.
Take out and let it cool down.
Roll out the dough on a floured work surface until round (approx. 30 cm Ø). Grease a springform pan (26 cm Ø) and dust with flour. Put the dough into it and press the edges firmly. Prick the dough base several times with a fork.
Place baking parchment on the dough, add dried peas. Bake blind in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 10 minutes. Remove baking paper with dry peas.
Bake the bottom for another 10 minutes. Remove from the pan and let cool lukewarm on a cake rack.
Whisk eggs with crème légère, season with salt. Crumble cheese. Spread red beets and cheese on the pastry base. Pour the egg icing over it and bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see
manufacturer) bake for about 30 minutes. Remove and let cool on a cake rack. Remove from the tin and arrange hot or cold. Garnish with thyme and possibly spinach. Sprinkle with pepper.