To make the hot pot
Heat the olive oil in a large frying pan over a medium heat. Add the onion and fry for 7 minutes, until softened. Add the garlic and cook for another 2 minutes, stirring.
Transfer the onion mix to the slow cooker pot with the celery, root vegetables, soup mix, herbs, the smaller quantity of hot stock and tomato purée, then stir until combined. Cover and cook for 6–7 hours on low or 5–6 hours on high, until the vegetables and soup mix are tender, adding more hot stock if needed.
If the stew needs thickening, 5 minutes before the end of the cooking time, mix the cornflour with a little cold water, then stir well into the slow cooker pot. Cover and continue to cook until thickened. Season to taste and serve in bowls, topped with an extra splash of olive oil and the carrot and parsnip crisps.
To make the carrot and parsnip crisps
Preheat the oven to 180°C/350°F/gas mark 4.
Place the vegetable peelings on a large baking sheet. Drizzle over a little olive oil and toss with your hands until the peelings are lightly coated in oil.
Spread them out, making sure they are not overcrowded. Season with salt. Bake in the preheated oven for 25 minutes, turning once or twice, until golden and crisp. Drain the on kitchen paper/paper towels and leave to cool and crisp up further before serving.