Preheat your oven to 400°F (200°C). Line a baking tray with parchment paper and lightly grease it with oil.
In a mixing bowl, combine the chickpea flour, cumin seeds, crushed coriander seeds, turmeric powder, salt, and chili powder.
Add the thinly sliced onions to the bowl and mix well to coat them with the flour mixture.
Gradually add water, a little at a time, until the mixture forms a thick batter that coats the onions. You may not need all the water.
Scoop small portions of the mixture and place them on the prepared baking tray, flattening them slightly with the back of a spoon.
Bake in the preheated oven for 20-25 minutes, or until the bhajis are golden brown and crispy, flipping them halfway through the baking time.
Remove from the oven and let them cool slightly before serving. Enjoy with your favorite chutney or dip.