Bring water to a boil in a medium saucepan. Add couscous, cover pot, and reduce heat to low. Simmer for 10 mins, then remove pan from heat, keep covered and rest a further 10 mins to finish cooking. Fluff couscous with a fork and transfer to a large bowl.
Whisk together orange zest and juice, mustard, vinegar, and olive oil. Season to taste with salt and pepper. Add half the dressing to the warm couscous and toss to combine.
Drizzle lamb chops with olive oil and sprinkle both sides with salt, pepper, and sumac. In a small bowl, whisk together apricot jam, honey, soy sauce, mustard, and garlic.
Preheat a large heavy-based skillet, or BBQ, to medium-high. Add chops and cook for 3 mins until golden. Turn chops over, brush with glaze and cook a further 2–3 mins.
Turn chops once more, brush with glaze, and immediately transfer to a clean plate. Cover loosely with foil and rest for 5 mins.
Add rocket, grapes, celery, walnuts, herbs, and remaining orange dressing to the bowl with the couscous, and mix well. Serve chops warm or at room temperature, with couscous salad.