Prepare the crispy onions by soaking them in buttermilk for 10 minutes.
Combine the flour, sea salt, freshly cracked black pepper, and garlic powder to taste, and mix well.
Dredge the onions by removing them from the buttermilk, shaking off excess liquid. Coat the onions evenly in the seasoned flour.
Heat the oil in a small skillet to 375 degrees using a thermometer.
Fry the onions in batches by dropping a few dredged onions into the hot oil and stirring often, until golden brown. Remove with a slotted spoon and drain on a paper towel-lined plate. Season with sea salt. Continue with remaining onions.
Marinate the tomatoes by placing the tomato slices on a few paper towels to blot them dry. Place on a plate and drizzle the slices with olive oil, red wine vinegar, sea salt, freshly cracked black pepper, and garlic powder to taste.
Cook the bacon in a large skillet until crispy and browned. Remove from the skillet and place on a few paper towels to drain.
Prepare the sandwich by slathering mayonnaise on one piece of rye bread. Top with cheese slices, crispy onions, roast beef, marinated tomato, bacon, and lettuce.
Slather the last piece of bread with creamy horseradish and top the sandwich.