Preheat the oven to 200°C/fan 180°C/gas mark 6. Cut the sweet potato, butternut squash, celeriac and carrots into evenly sized thick-cut chips. Put these in a large roasting tray with the garlic cloves. Season with black pepper, tuck in the thyme sprigs and drizzle over the olive oil. Roast in the oven for 40-45 minutes until soft and golden. Squeeze the garlic cloves to release the purée and stir through the vegetables.
Meanwhile, brush the steaks on both sides with the mustard. Put the crushed black pepper on a plate and turn the steaks in the pepper. Heat the oil in a large frying or griddle pan and cook the steaks for 2-3 minutes on each side, depending on how well done you like them. Leave to rest for a minute, then serve with the vegetable crisps on the side.