Preheat oven to gas mark 4 / 177*C / 350F.
Open up a Large Bakewell oven bag.
Add to the bag: 3 chopped tomatoes, 2 chopped carrots, 2 chopped bell peppers, 1/2 a head of cauliflower chopped into florets, 1/2 head of broccoli chopped into florets and 1 chopped celery head.
Add 1 teaspoon of olive oil and salt and pepper.
Close the bag loosely with one of the ties provided.
Cut the top corner of the bag using a scissors.
Place in an ovenable dish in the middle of the oven.
Cook for 30-40 minutes, or until cooked.
Carefully open the bag and serve.