Juice the orange. Pour the juice into a food processor bowl. Add half of the watercress. Blitz with a pinch of salt and pepper. (If you don’t have a processor, finely chop the watercress and mix with the orange juice.) Set aside for later.
Tip the quinoa into a sieve. Rinse under cold water for 1-2 mins to remove any bitterness. Tip into a pan and cover with 200 ml boiling water. Place on a medium heat. Cook for 15 mins till the water is absorbed and the quinoa is tender.
Meanwhile, peel and coarsely grate the sweet potato. Trim the leek and slice it as finely as possible. Tip both into a bowl.
Crack 1 egg into a small bowl. Squeeze in 2 tsp of the Dijon mustard. Beat together with a pinch of salt and pepper. Tip the quinoa into the bowl with the sweet potato and leeks. Stir for 1-2 mins to cool it down. Pour in the Dijon and egg mix. Stir well to mix.
Scoop up 1 tbsp of the mixture. Keeping it on the spoon, press it down with another tablespoon to compress it. Carefully push the ‘cake’ off the spoon onto a baking tray lined with baking paper. Repeat with the remaining mixture till you have 6-8 cakes. Slide the tray under the grill for 5 mins.
Carefully flip the sweet potato and quinoa cakes over. Slide back under the grill for a further 3-5 mins till they look firm and are a little charred at the edges.
Bring a pan of water to a very gentle boil. Crack 1 egg into a small cup. Carefully slide it into the water. Repeat with the other egg. Cook for 2-3 mins for a runny yolk or 3-4 mins for a more set yolk.
Stack a few sweet potato cakes on 2 warm plates. Add the poached egg. Scatter over the watercress and drizzle over the watercress and orange dressing to serve.