Before you start, preheat oven to 220°C (or 200°C fan bake). Preheat BBQ grill to medium-high (if using).
Prep cauliflower and onion and toss on a lined oven tray with sumac seeds and salt. Roast for about 20 minutes, until tender. Remove from oven and allow to cool slightly.
Pat salmon dry and remove any remaining pin bones. Cut into 2 even pieces. Season with salt and coat flesh in chermoula, avoiding the skin.
Heat oil in a medium fry-pan on medium-high heat. Cook salmon, skin side down, for about 3 minutes, until skin is crispy. Use a fish slice to press on fleshy side of salmon to ensure all skin is touching the pan. Flip and cook for a further 2 minutes for medium, or until cooked to your liking. Alternatively, cook salmon on BBQ.
Prep courgette. In a medium bowl, toss cooked cauliflower with remaining salad ingredients. Season to taste with salt and pepper. Prep grapes.
Serve cauliflower salad with salmon, yoghurt and grapes.