Preheat the oven to 200°C.
Divide oil between two trays and add beetroot with one garlic clove to one, and carrots with another clove to the other.
Roast vegetables for 20 minutes until cooked through and golden-brown.
Drizzle with sherry vinegar, sprinkle with thyme, and season with salt and pepper.
Heat 1 tablespoon of oil in a frying pan and soften the onion for 4-5 minutes.
Sift flour and baking powder into a bowl and mix in the fried onion.
Mix in butter, eggs, milk, mashed potato, parmesan, and seasoning to form a dough.
Roll the mixture on a floured surface to 2.5cm thickness.
Cut out six 7.5cm diameter circles from the dough.
Fry the potato cakes in oil for 3-4 minutes per side until golden.
Serve cakes with roasted vegetables, roasting juices, watercress, and pomegranate seeds.