Preheat the oven to 180°C.
Cut the beets and onion in half (don’t peel them) and put them in a baking dish. Tuck the garlic between them (again, no need to peel them). Drizzle with the oil, then cover the tray with foil.
Bake in the oven for 30 minutes or until the veggies are soft. Let them cool in the dish.
When the veggies have cooled down, remove the skins and rough parts. Push the garlic flesh out of its skin. (At this point, you can chill the peeled veggies for later use).
Put the peeled vegetable in a saucepan. Add the stock and the green peas and bring to the boil. Simmer for a couple minutes, then turn off the heat. Add the dill and, using a stick blender, blitz the broth into a thick, smooth, velvety soup. Season with salt and pepper, if needed.
Add a spoonful of cider vinegar, then serve.