Heat your oven to 200ºC/Fan 180ºC/Gas 6. Line a baking tray with greaseproof paper. Brush with ½ tbsp olive oil. Crack the egg into a bowl and beat. Rinse the parsley. Pat the dry. Roughly chop the stalks. Save the leaves for later. Add the stalks to the egg. Season.
Halve the celeriac. Peel it with a small, sharp knife. Coarsely grate it. Pop it in the bowl with the egg. Add 5 tbsp of the flour and 1½ tbsp olive oil. Stir well to mix everything together.
Shape the celeriac mixture into 6 patties. The mixture may feel a little loose but it will hold together. Pop each rösti onto the lined baking tray, spacing well apart.
Bake for 10 mins before carefully turning them over using a fish slice or spatula. Press the mixture down a little to help firm them up. Repeat 3 more times every 10 mins till the röstis are browned and crisp at the edges.
While the rösti’s cook, make the ratatouille. Peel and chop the onion, carrot, parsnip and squash into chunky bite sized pieces. Peel and grate or crush the garlic. Heat 1 tbsp oil in a pan. Add the veg. Cook for 8-10 mins till glossy and soft. Pop the garlic in for the last 1 min.
Drain and rinse the chickpeas. Add to the veg along with the tomatoes. Half fill the tomato can with water and add that too. Cover and simmer on the lowest heat for 15–20 mins till the veg is tender. Stir in the balsamic vinegar.
Taste the ratatouille. Adjust the seasoning if you think it needs it. Roughly chop the parsley leaves you saved in step 1.
Carefully lift the röstis on to warmed plates. Top with the ratatouille. Scatter with chopped parsley to serve.