Preheat oven to 220 C / fan oven 200 C / gas 7.

Cut cauliflower in small florets and Brussels sprouts in halves (both should have similar sizes to cook evenly). Spread on an oven tray. Add peeled garlic cloves and drizzle with oil.

Roast in the oven for 25-30mins until golden and cooked.

When the vegetables are done, heat 2 Tbsp of olive oil in a large pan. Add finely chopped onion and cook under medium heat until soft. Add roasted vegetables, rosemary, vegetable stock and water. Bring to the boil, then cook covered under medium heat from 20 mins.

When done, whizz the soup to desired consistency, adding more vegetable stock or water if necessary. Season to taste, with lemon juice.