Your problem is that the meat and veg aren't enough to give the casserole a tasty flavour. Multiple oxo cubes are not the answer!
You need a gravy or sauce. Plenty of suggestions above or just google recipes.
A good recipe is best but if you are looking for short cuts better than 8 Oxo cubes here's some suggestions:
Gravy made with a heaped tablespoon of Bisto gravy granules added to boiled water with a dash of red wine
Half a pint of stock made with one Oxo cube with quarter of a pint of boiling water and quarter of a pint of red or white wine
Half a pint of stock made with one Oxo cube with added flavours: choose some from tomato purée, chilli powder, balsamic vinegar, chutney, spices e.g. cumin, paprika, herbs e.g. tarragon, rosemary
Packets of gravy or casserole mixes or jars of casserole mixes
But really, start searching for recipes and experiment till you find some you all like