Cut the potatoes and onion into wedges and the carrots and parsnips into quarters. Put in a large roasting tin to fit in a single layer. Add the garlic cloves, thyme sprigs, oil and salt and pepper and stir well until evenly coated. Roast in a preheated oven, 220°C (425°F), Gas Mark 7, for 50 minutes–1 hour until browned and tender, stirring halfway through.
Meanwhile, make the pesto. Heat a heavy-based frying pan until hot, add the almonds and cook over a medium heat, stirring, for 3–4 minutes until browned. Transfer to a bowl and leave to cool.
Put the almonds in a mortar or food processor, add the parsley, garlic and salt and pepper and grind with a pestle or process to form a coarse paste. Transfer to a bowl, stir in the oil and Parmesan and adjust the seasoning.
Serve the roast vegetables hot with the pesto.