The white and light green parts of leeks can be used as a substitute for onions. They have a milder, slightly sweet flavour.
While garlic has a different flavour profile, it can still add depth to dishes in the absence of onions. Use it in combination with other substitutes.
You can use asafoetida (hing) in small amounts for a distinct, pungent flavor that somewhat mimics the umami of onions.
Ginger is another essential ingredient in Indian cooking, providing a distinct, warm, and spicy flavour.
Bell peppers, with their sweet and mild flavor, can also be substituted in Indian dishes.
Fennel has a mild, sweet, and slightly anise-like flavor. It can be used in place of onions in recipes where its unique flavor complements the dish.
When cooked down, cabbage can mimic the texture of onions. It has a milder flavor but can work well in curries and stews.
Onion flakes are a convenient and effective alternative to fresh onions in Indian cooking. They are made from dehydrated onions.