Preheat your oven to 200°C/Fan 180°C/Gas 6. Fill and boil your kettle.
Rinse the pearl barley and tip it into a medium pan. Pour in enough boiling water to cover the barley by a few cms and add a pinch of salt. Bring back to the boil, then turn the heat down, cover and simmer for 20-25 mins, till the barley is tender. Drain and tip it back into the pan.
While the barley cooks, peel the parsnips and chop them into 2cm-thick pieces. Slice 1cm off the pointy end of the garlic bulb. Tip the parsnips into a roasting tin and add 1 tbsp olive oil and a good pinch of salt and pepper. Toss to mix, then pop the garlic bulb in the centre of the veg. Roast for 20 mins, turning halfway through cooking.
Meanwhile, whisk 1 tsp Dijon mustard in a small bowl with 1 tbsp each honey and olive oil. Season with salt and pepper and set aside.
Remove the parsnips from the oven. Carefully lift the garlic bulb out of the tin and let it cool a little. Squeeze the roast garlic cloves out and fold them through the drained pearl barley.
Grate the lemon zest over the parsnips and add a pinch of chilli powder and the thyme leaves. Toss to mix, then tip in the barley and fold together. Pour over the honey and mustard dressing, add the lamb's lettuce and toss again. Divide the salad between 2 plates and serve.