For the onion ring masala: Combine the chaat masala, ground cumin, chili powder and mint.
Mix together the buttermilk, tandoori masala and 2 teaspoons of the onion ring masala. Add the onion rings and press to coat. Marinate covered in the refrigerator for at least 1 hour.
Fill a large Dutch oven halfway with vegetable oil and heat to 375 degrees F.
Mix together the flour and 2 tablespoons onion ring masala in a shallow bowl or large dish. Dredge the onions in the seasoned flour in batches, coating each ring evenly. Shake off any excess flour and carefully place the rings into the hot oil. Fry in batches until golden brown, turning to cook evenly and prevent sticking, about 3 minutes. Remove from the oil and drain on paper towels.
Season the fried onion rings with more onion ring masala and salt, and repeat with the remaining onions. Make sure the vegetable oil comes back up to 375 degrees F in between batches.