Preheat your oven to 200°C/Fan 180°C/Gas 6. Chop the tomatoes and tip them into a roasting tin. Peel and finely chop the onion and add to the tomatoes. Drop in the unpeeled, whole garlic cloves. Drizzle in 1 tbsp olive oil and add some salt and pepper. Toss together. Slide into the oven for 30 mins.
While the tomatoes roast, finely chop the rosemary leaves. Finely grate the zest from the lemon.
When the tomatoes have 5 mins left to cook, fill and boil the kettle. After 30 mins, remove the tomatoes from the oven and pluck out the garlic cloves, setting them to one side.
Pour 800ml hot water from the kettle into the tin with the tomatoes and crumble in the stock cube. Place on a medium heat on the hob (alternatively, transfer the roast tomatoes to a pan).
Bring the soup up to a bubble then sprinkle in the rosemary leaves. Squeeze in the roasted garlic flesh. Simmer together for 5 mins. Using a blender, blitz the tomatoes till smooth.
Bring the soup back up to a simmer then carefully drop in the tortelloni. Cook for 4-5 mins till the pasta is tender. Add the lemon zest and squeeze in the juice from half the lemon. Stir and taste, adding more salt, pepper or lemon juice if you think it needs it. Ladle the soup into bowls, dividing the tortelloni. Crack over some black pepper and drizzle over 1 tsp olive oil over each.