Heat your oven to 200°C/Fan 180°C/Gas 6. Tumble the tomatoes into a roasting tin. Trim the dry ends off the fennel and pinch off any soft fronds. Set the fronds aside for later. Halve the fennel lengthways, then slice each half into 6 wedges. Peel and slice the onion into around 8 wedges. Add the fennel and onion to the tin.
Drizzle the veg with 1 tbsp olive oil and season with salt and pepper. Toss the veg to coat them in the seasoned oil, and sprinkle with 1 tsp fennel seeds. Roast the veg for 20 mins.
Drop the garlic cloves into a corner of the tin. Return the veg to the oven and roast for 20 mins more, till they are tender and lightly charred.
While the veg roast, roughly chop the parsley leaves. Crumble the stock cube into a heatproof jug and stir in 400ml boiling water till it dissolves.
When the veg are roasted, remove them from the oven, picking the garlic cloves out of the tin. Tip the veg into a blender and squeeze the garlic flesh in too. Pour in the stock and blitz till smooth. Alternatively, you can use a hand-held blender in the roasting tin, making sure to squeeze the garlic cloves from their skins before adding the stock.
Taste the soup and add a little more salt or pepper if you think it needs it. Ladle into warm bowls and serve garnished with chopped parsley.