Heat 3 inches of vegetable oil in a large Dutch oven or heavy-bottomed pot over medium-high heat until a deep-fry thermometer registers 365 degrees F. Meanwhile, preheat the oven to 200 degrees F and set a rack on a rimmed baking sheet.
Whisk 1 cup all-purpose flour, the rice flour, baking powder, steak seasoning, 1/2 teaspoon salt and a few grinds of pepper in a large bowl. Whisk in the beer and seltzer until mostly smooth. Let sit 5 minutes to thicken. Place the remaining 1/2 cup all-purpose flour in a shallow bowl.
Working in about five batches, dredge the onion rings in the flour, then dip in the beer batter, letting the excess drip off. Fry until crisp and golden brown, flipping occasionally, 3 to 4 minutes. Remove to the rack using a slotted spoon, season with salt and keep warm in the oven while you fry the remaining batches.