Substitute for onions and garlic
Many South Asian recipes use a (fairly weird) spice called "hing" or asafoetida powder. It's got a vaguely similar aroma.

Generally you add a pinch to hot oil before beginning a dish. A half-teaspoon is usually enough for a four- to six-serving preparation.

You can find it quite easily at South Asian grocery stores and online.

Oh, and it's "weird" because of the way it's gathered. Unlike other spices, it's not from a seed or seed pod or bark &mdash; it's sap, dried and ground. (Most hing powders are packaged with some filler.)