It should be a very light, weak soup – after all, it is always boiled down again until the risotto is cooked, and a dense, spicy beef soup could then turn out too strong and intense.
You don’t need more than a piece of leek, a carrot, an onion, maybe a few steps of the celeriac and some parsley stems: Cover with water, add a bay leaf or two, a tsp of peppercorns, maybe a few mustard seeds and allspice berries. No salt! Silently simmer gently for at least an hour, perhaps pouring fresh water and adding more vegetable trimmings.