Heat your oven to 180°C/Fan 160°C/Gas 4. Scrub the sweet potatoes and slice them in half lengthways, then halve again them same way to make thick oval slices. Slice a criss-cross pattern into one side of the sweet potato slices, cutting around 1/2cm deep. Brush ½ tbsp olive oil over the sweet potatoes, then place them on a baking tray, oiled-side up, and roast for 25 mins till tender.
While the sweet potatoes roast, make the sweet miso glaze. Spoon the brown rice miso paste into a bowl and add 1 tbsp caster sugar and 2 tbsp brown rice vinegar. Stir together to make a thick, sticky paste. Set aside.
Trim the roots off the spring onions and trim away any ragged leaves. Thinly slice the spring onions, white and green parts, but keep the white and green parts separate. Tip the quinoa into a sieve and rinse under cold water for 1-2 mins – this washes off any soapy coating left on the grains, which can make them taste bitter.
Warm a small pan over a medium heat for 1 min, then add ½ tbsp oil and the white parts of the spring onion. Season with a little salt and fry, stirring for 3-4 mins till the spring onions are soft.
Tip in the quinoa. Pour in 300ml boiling water. Stir to mix, pop on a lid and bring to the boil. Once it's boiling, turn the heat down and simmer for 12-15 mins till the quinoa is tender and has absorbed the water. When the quinoa is cooked, take the pan off the heat and stir in most of the sesame seeds. Leave the quinoa in the pan to keep warm.
When the sweet potatoes are tender, take them out of the oven. Set your grill to high. Brush the sweet miso glaze over the top of the sweet potato slices. Slide under the grill for 3-5 mins till the glaze is bubbling and dark brown and the potatoes are a little charred.
Toss the salad leaves with 1 tsp oil and a pinch of salt and pepper. Slice the lime into wedges.
Spoon the quinoa onto 2 warm plates, top with the salad leaves and miso-glazed sweet potatoes. Sprinkle over the last of the sesame seeds, the spring onion greens and serve with the lime wedges for squeezing.