In a medium saucepan, toast the barley in butter on medium heat, for about 5 minutes until it begins to brown. Add the stock, reduce the heat to low. Cover and cook for about 30 minutes or until barley is tender and liquid is absorbed. Season with salt and pepper. Keep warm.
In a pan, saute the onion, carrot, red pepper, zucchini, celery, garlic, and chives in olive oil and butter for about 10 minutes, until golden brown. Combine the vegetables with the barley. Season with salt and pepper. Keep aside.
For the mustard sauce:
Add olive oil and saute the shallots for 1 minute. Add the capers, stock, and mustard. Continue cooking for 10 to 15 minutes, until reduced by 1/2. Add the butter and whisk until fully incorporated. Season with salt and pepper. Keep warm.
For the sole filets:
Preheat the oven to 350 degrees F.
Put the sole fillets on a baking tray covered with parchment paper. Season with salt and pepper and a drizzle of olive oil. Cook in the oven for 5 minutes.
When ready to serve, plate the barley with the sole filets, garnish with the sauce and chive.