Heat a large saucepan over a medium heat. Add 25g of the butter and, once melted, add the onion and squash along with a pinch of salt. Gently fry for 10 minutes, or until the onion is translucent and the squash is softening. Add the garlic and continue to cook for two minutes, or until aromatic.
In a separate pan, warm the stock and half of the sage leaves.
Add 25g of the butter to the onion and squash and, once melted, add the pearl barley. Turn up the heat and cook for 2-3 minutes. Turn down the heat to medium and add a ladleful of warm stock. Stir well to mix and, once absorbed, add another ladleful, stirring now and again. Repeat this process until the stock is used up and the pearl barley is tender (about 30 minutes).
Once the barley is tender, taste and season to taste with salt and pepper. Add the remaining butter to the pan and stir through.