Preheat the oven to 180°C.
Peel, then cut the beets into wedges.
Cut the onions in wedges too. Lightly crush the garlic cloves (with their skin on).
Cut the pepper into chunks.
Arrange all of them in a baking tray. Tuck the chilli pepper between them (if used), then scatter the oregano on top.
Drizzle with the oil and 2 Tbsp balsamic vinegar, then season with salt & pepper. Toss gently.
Cover the dish with foil, then bake in the oven for 25 minutes. During the baking, toss the veggies once or twice. Take the foil off and roast for 5 minutes.
When the veggies are tender, remove the dish from the oven.
Let the veggies cool a bit, then discard all the skins.
Put the flesh of all veggies in a saucepan and add the tinned tomatoes.
Using the tin, add some water to the pan, together with the stock cube.
Bring to the boil, then add the remaining balsamic vinegar and reduce the sauce until it’s thick and a bit sticky.
Taste and season with salt and pepper to your liking.