Tip the dried porcini into a mug or heatproof bowl. Cover with 300 ml boiling water. Peel and finely dice your onion. Scrub or peel your carrot and finely dice it. Trim your celery and finely dice it.
Warm a pan for 1 min. Add 1 tbsp oil and the veg. Add a good pinch of salt and pepper. Cover, turn the heat down and sweat for 8-10 mins till glossy.
Peel and chop your garlic. Strip the leaves from 2 rosemary sprigs. Finely chop the leaves. Stir the garlic and rosemary into the veg.
Add the chopped tomatoes, the porcini and the porcini soaking liquid. Simmer for 10 mins. Ladle the soup into a blender and blitz till smooth, or use a hand-held blender. If it's too thick, add a splash more water. Taste and add more salt and pepper if it needs it. Serve in warm bowls, garnished with a few rosemary leaves.