Place the millet couscous, corn couscous and fonio (or quinoa) into three separate bowls and cover with water. Soak for 20 minutes, then drain. (If you are using quinoa, you will need to par-boil this for ten minutes and drain thoroughly before steaming.)
Place the grains all together in a steamer and steam for 20 minutes, adding water and cooking until the grains are thoroughly cooked through. Add the butter and remove from the heat.
Heat the oil in a pan and gently fry the meat until browned all over (you may need to do this in batches). Add the tomato purée, passata, a little water and salt and pepper to taste to the pan with the beef. After about ten minutes, add the chopped vegetables and the crushed garlic.
Cook for about 15 minutes or until the vegetables are cooked but firm and the sauce has thickened.
Serve the couscous with the meat and vegetable sauce.