If you have refrigerated your steaks, take them out at this point and unwrap them from their packaging. You want the ribeye steaks to be at room temperature before you grill them.
Meanwhile, preheat your charcoal grill over medium heat and prep your beer-battered onion rings.
To a large and shallow bowl, add the sliced onion rings. Pour the buttermilk over top of the onions and set aside.
In one large bowl, add 1 1/2 cups all-purpose flour combined with a dash of salt and black pepper. To a second large bowl, whisk together the remaining 1 cup all-purpose flour, brown rice flour, seltzer water, beer and honey.
To a large, deep dutch oven, add at least 3 inches of oil. Using a frying thermometer, heat the oil until it reaches a temperature of 350 degrees F.
While the oil is heating up, prepare your mustard sauce. Whisk the mayonnaise, honey mustard, horseradish, honey, cinnamon, salt, black pepper, and cayenne pepper in a small bowl. Set aside.
Working in batches, transfer 6-7 buttermilk-soaked onion rings into the bowl with just the flour. Shake off any excess and transfer the rings to the batter. Shake off any excess. Transfer the rings immediately to the hot oil and fry for about 3 minutes on each side, or until golden brown. Transfer the now golden brown onion rings to a plate that has been lined with a paper towel. Repeat this step for the rest of the onion rings. To keep them warm, set your oven to 125 degrees F and transfer the rings to a baking sheet. This will keep them warm, while you prepare your steak.
In a small bowl, whisk together the chili powder, kosher salt, and black pepper for the ribeye steaks. Rub the mixture all over the two steaks. Using a pair of tongs, transfer the steaks to the hot grill. Grill for about 4 minutes on each side for medium-rare or grill until the steak is done to your liking. Use a meat thermometer for accuracy.
Remove the steaks from the grill and set aside to rest for at least 8 minutes before serving. Serve the ribeye steaks topped with beer-battered onion rings, flaky sea salt, and fresh parsley. Enjoy!