Bring a pan of salted water to the boil. Tip in the pearl barley. Cover, bring back to the boil, then turn the heat down. Simmer for 20-25 mins till the barley is tender and cooked through.
While the barley simmers, put a dry frying or griddle pan on a medium heat. Add the sunflower seeds and toast for 2-3 mins till nutty smelling and browned. Tip into a bowl and set aside.
Snap the ends off the asparagus (bend the spears, they'll snap at the right point), then slice into 3cm lengths. Trim the spring onions. Halve them widthways, then lengthways, to make 8 chunks. Halve the nectarines and scoop out the stones. Slice into around 8 thick wedges each.
Juice half the lemon into a small bowl. Add 1 tbsp oil, 1 tbsp mustard and some seasoning. Whisk to make a dressing. Taste and add more salt, pepper, lemon juice or olive oil if needed.
Put the dry pan back on the heat. Warm for 2 mins. Add the asparagus and spring onions. Fry for 3-5 mins, turning occasionally, till lightly charred. Scoop them into a large bowl.
Add the nectarines to the pan. Fry for 4-5 mins till browned, then flip over and fry for 3-4 mins. Add to the bowl.
Add the lamb's lettuce to the bowl, too. The pearl barley should be cooked by now, so drain it and tip into the bowl with the sunflower seeds. Pour in the dressing and toss to mix. Pile the salad onto 2 warm plates and serve straight away.