Try cooking carrots, butternut squash, onions, white cabbage, cauliflower, broccoli and celery until they are really soft. These veg are all very mushy when overcooked so they should blend easily without any lumps. Add a carton of passata. Season to taste. Add some tomato puree to enhance the flavour if its not tomatoey enough for you. (not sure if tomato puree is free or syned).