I lean heavily on onion and garlic as I enjoy the flavors, so I would miss them.
However there are many ways to enhance flavor with other things like mushrooms (dried especially - porcini, shiitake, etc), tomato paste, cross-cultural additions like miso / doenjang / gochujang / fish sauce, spices, and so on.
This Braised brisket with porcini gravy from the current COTM (Nigel Slater) sounds really good – it has shallots, but you could sub leeks:
