Tip the quinoa into a sieve and rinse under cold water. Tip into a pan. Peel and finely chop the sweet potato and add to the pan. Cover with lots of boiling water. Add a pinch of salt, cover and bring to the boil. Turn the heat down and simmer for 15 mins till the potato and quinoa are softening.
Meanwhile, trim, rinse and finely slice the leek. Set aside. Pick any thick stalks from the watercress and tip the tender leaves and shoots into a bowl. Slice the skin and pith off the orange. Slice it into segments. Add to the watercress. Set aside.
When the potato and quinoa have boiled for 15 mins, add the leek and simmer for 5 mins till the potato is very soft. Drain. Rinse under cold water and drain again. Tip into a bowl. Add 1 tbsp cornflour. Mash together.
Crack 1 egg into a small bowl. Add 1 tsp mustard. Beat together with a pinch of salt and pepper. Add to the bowl and stir to mix. Set aside to cool a little.
Heat your grill to high. Place the tomatoes on a baking tray and slide under the grill. Cook for 8 mins, till bursting and a little charred.
Meanwhile, whisk the remaining mustard with 1 tbsp cider vinegar, 2 tsp olive oil and a pinch of salt and pepper. Toss with the watercress and orange. Put a pan of water on to boil.
When the tomatoes have finished cooking, set them aside. Line the baking tray or grill pan with foil. Brush with 1 tsp oil. Scoop 1 large tbsp out of the sweet potato mixture and pat into a round. Pop on the tray. Repeat with the rest of the mix. You should have 6-8 rounds in total. Lay them on the baking tray. Grill for 5-8 mins, till browned, then use a spatula to carefully turn them over and grill for another 5-10 mins till golden.
While the potato cakes grill, the water in the pan should boil. When you're nearly ready to serve, set the heat to medium-low so it's just boiling – you only need a few bubbles. Crack 1 egg into a small cup. Carefully slide it into the water. Repeat with the other egg. Gently cook for 2-3 mins for a runny yolk or 3-4 mins for a more set yolk.
Arrange the watercress salad and grilled cherry tomatoes on 2 warm plates. Pile the sweet potato cakes up on the plate. Top with a poached egg each and serve.