You can use: Worcestershire sauce just a couple of dashes in the cooking liquid will do. Make vegetable stock with celery stalks and leaves, carrot, onion, chilli, sweet peppers; mixed herbs, simmer for about an hour and use the liquid; avoid brassicas in stock. Squeeze tomato puree into the cooking liquid. Don't bother, use just plain water to cook the casserole it will be flavoured from the beef and vegetables you include. Include a pinch of Maldon sea salt for extra flavour.
I often substitute vegetable stock for beef stock if I don't have it. I also just add more depth of flavor with Worcestershire sauce. You could also use a beef bullion cube to add more of a beef flavor.