Before you start, preheat BBQ grill to medium (if using).
Add couscous and spices to a small, dry pot and cook for 1-2 minutes, until fragrant. Add stock, salt and oil and bring to the boil. Once boiling, turn off heat, cover and leave for 5 minutes, then fluff up grains with a fork. Set aside.
Prep cucumber and onion. Zest orange, then cut flesh into 1cm segments. Add all to a medium bowl along with spinach.
Prep feta and mint.
Pat lamb dry, season with salt and pepper and toss with oil. Cook lamb on BB grill for 7-10 minutes each side for medium-rare (depending on thickness), or until cooked to your liking. Rest, covered for 5 minutes. Alternatively, cook lamb in a medium fry-pan on medium-high heat.
To finish, add cooked couscous to bowl with salad, toss to combine and season to taste with salt and pepper. Slice lamb thinly against the grain and season with salt and pepper. Mix pistachios and sumac together, then sprinkle over lamb and toss to coat.
Serve couscous salad topped with lamb, feta, yoghurt and mint.