Mediterranean-style roasted red pepper soup

3 red peppers, chopped

2 red onions, chopped

3 garlic cloves, chopped

1 tsp coriander seeds or ground coriander

salt

black pepper

2 tbsp olive oil, plus extra for cooking

2 celery sticks

1 x 400g tin of chopped tomatoes

1 vegetable stock cube

300ml boiling water

fresh basil leaves, to garnish