Preheat the oven on 428 degrees Fahrenheit or 220 degrees Celsius.
In the meantime you can cut the beets and onions in quarters. Cut the carrots, celery sticks and leek in big even pieces.
Add the beets, olive oil and some salt and pepper to the oven dish. Roast the beets for about 15 minutes. Then add the onions, carrots, celery sticks and leek. Roast everything for another 25-30 minutes.
Cooking
Transfer the roasted vegetables from the oven dish to a large cooking pot. Add the tomato puree, bay leaves, thyme, clove, peppercorns and salt. Pour in 2,5 liters of water and bring it to a boil. Let it cook for about 3 hours.
You can of course let it cook for a shorter amount of time. However this will impact the complexity in taste ;)
Place a fine sieve on a bowl and pour the stock through it. Let the stock cool down. It will stay good for about 2 days in your refrigerator.
Preferring to store it for a longer period? Pour the stock in an ice cube tray and freeze it. It will stay good for about 5 months.