1 large yellow onion, peeled and cut into 1/2-inch thick rounds
¼ cup (30g) all-purpose flour
1 cup (125g) chickpea flour
¼ cup (60g) cornstarch
1 teaspoon (4g) ground turmeric
1 teaspoon (4g) ground cumin
½ teaspoon (2g) cayenne pepper
½ teaspoon (2g) ground coriander
½ teaspoon (2g) garam masala
½ teaspoon (2g) kosher salt
½ teaspoon (2g) baking powder
¼ teaspoon (1g) mustard powder
¾ cup (177ml) water
Heat oven to 200F (93C) and line a large rimmed baking sheet with a cooling rack; set aside.
Heat 1½-inches (4cm) of oil in a large, heavy bottomed saucepan or Dutch oven to 350F (176C).
Separate onion slices into rings. Toss rings into a large bowl with all-purpose flour to coat.
Whisk together remaining ingredients except water in a large bowl. Whisk in water until combined. Dip rings into the batter one at a time and add to oil. Fry in batched of 5 or 6 until golden brown and crispy, 3 to 4 minutes, flipping halfway and adjusting heat up or down to maintain temperature of oil.
Remove the rings to prepared pan and sprinkle with additional salt. Place in oven to keep warm and fry remaining rings.