Mix flour and ½ tsp. salt in a bowl. Crumble the butter in.
Work it into a dough. Add a little water if the dough doesn't come together.
Press the dough into a tart tin (about 28 cm in diameter). Put it in the fridge for 20 minutes. You can also buy a ready-made tart dough.
Prick the tart dough with a fork.
Bake it at 200 degrees Celsius (392 degrees Fahrenheit) for 10 minutes.
Peel the beetroots. Grate them coarsely.
Chop the thyme. Cut the onion into thin wedges.
Heat the oil in a pan. Add the onion, thyme, and balsamic vinegar. Fry for 5 minutes.
Put the onion mixture in the tart tin. Spread the beetroot over.
Crumble the goat cheese over the beetroot.
Whisk eggs, cream, honey, 1 tsp. salt, and pepper together.
Pour the mixture over the tart.
Bake at 200 degrees Celsius (392 degrees Fahrenheit) for 35-40 minutes.
Serving.