Peel the red onions and thinly slice them. Finely chop the rosemary leaves. Peel the carrot and finely dice it.
Pour 1 tbsp olive oil into a pan and warm to a medium heat. Slide in the sliced onion along with the chopped carrot and rosemary. Sprinkle in a little salt and pepper. Cover with a lid and cook for 12-15 mins, stirring every so often, till the onions are very soft and caramelised at the edges.
While the onions are cooking, scrub and peel the potato, and cut it into small chunks. Slide the potato into the pan. Crumble in the stock cube. Pour in 750ml boiling water. Return to the boil, then reduce to a simmer and cook for 10-15 mins, till the potatoes have softened.
Meanwhile, finely shred the wild garlic leaves. When the potatoes are tender, stir through the wild garlic.
Taste the broth and add more salt and pepper if needed. Ladle into a couple of bowls and serve.