Tip the pearl barley into a sieve and give it a good rinse under cold water. Tip it into a small pan. Pour in 300ml boiling water. Add a pinch of salt. Cover and bring to the boil.
Once the pan of pearl barley is boiling, turn the heat down to low and simmer for 20-25 mins till the water is absorbed and the pearl barley is tender. If the pan looks like it's drying out too quickly, add a splash more water.
While the barley simmers, peel and slice the onion into wedges. Halve the pepper, scoop out the seeds and white bits and slice it into long lengths. Chop the courgettes into batons around 3cm long.
Put all the veg in a bowl. Add 2 tsp olive oil and some salt and pepper. Turn all the veg to coat in the seasoned oil. Set aside.
Finely grate the zest from the lemon into a small bowl. Squeeze in the juice from half the lemon. Peel and crush or grate the garlic and add it to the lemon juice. Stir in 1 tbsp mustard with 1 tbsp olive oil and a pinch of salt and pepper. Taste and add more seasoning if you think it needs it. Set the lemon dressing to one side.
Pick any thick stalks out of the watercress so you have just the leaves and tender sprigs. Pop the watercress into a large bowl and put to one side.
Warm a large frying pan for 1 min. Add the veg. Stir fry for 5-8 mins till the veg are tender and charred at the edges. Tip the veg into the bowl with the watercress. Drain off any excess water from the pearl barley and tip the barley into the bowl. Pour in the lemon dressing and toss to mix well.
Put the frying pan back on the heat. Season the steaks with salt and pepper. When the pan is really hot, add the steaks. Fry for 3-5 mins on each side, depending on how well done you like them. Lift them out of the pan and rest for a few mins. Slice the steaks and serve them with the pearl barley salad on the side.