Preheat oven to 190 degrees Celsius/fan mode.
Cover a baking sheet with parchment paper.
Cut a piece of aluminum foil and sprinkle it with coarse salt in the center.
Place a beetroot head on the coarse salt, wrap in packet and transfer on the baking sheet.
Repeat until no beetroot heads are left.
Roast in the oven until tender, but crisp, for approximately 45-50 minutes.
Remove the baking sheet from the oven.
Open the foil packets and let the beetroots to cool for a while, until you can handle them.
Peel, cut into quarters and transfer them in a bowl.
In the bowl, add the onion and a little bit of olive oil and vinegar.
Toss gently and keep aside.