Put the pearl barley and stock cube in a saucepan and pour in 750ml/1¼ pint boiling water from the kettle. Stir well and bring to the boil. Reduce to a gentle simmer and cook for 35 minutes, until tender. When the barley is 3 minutes or so from being ready, add the peas to the pan and cover with a lid. Cook for 3 minutes, then drain well.
Meanwhile, heat the oil in a frying pan over a medium–high heat. Add the onion and cook, stirring, for 3 minutes. Add the garlic and cook for 30 seconds. Chop the squash into small bite-sized pieces and add to the pan. Stir-fry until heated through. Add the spinach and cook until wilted. Add the barley mixture and season with salt and pepper.
Divide into two portions. If following the meal plan, keep one portion in a sealed container in the fridge to eat for tomorrow's lunch (see Recipe Tip). Serve garnished with fresh basil.