Preheat your oven to 200C/Fan 180C/Gas 6.
Trim the onion squash and cut in half. Scoop out the seeds with a spoon and discard. Cut the flesh into wedges. Halve the red pepper and scoop out the seeds and white pith. Cut into thick slices. Slice the tomatoes into wedges. Peel the onion and slice into wedges.
Scatter all of the sliced vegetables onto a large roasting tin and sprinkle over the ground cumin and a good pinch of salt and pepper. Pour over 1 tbsp oil and give everything a good toss to evenly coat. Tuck in the unpeeled clove of garlic. Slide the vegetables into the oven and roast for 25 mins, turning halfway.
When the vegetables are roasted and tender, tip them into a large pan. Pour in 600ml boiling water and crumble in the stock cube. Stir well to dissolve. Use a stick blender to whiz the vegetables into a smooth soup. No stick blender? No problem. Simply transfer the vegetables and stock (in batches if necessary) into a food processor and blend.
Ladle the soup into deep bowls and serve.