For the creamed spinach: Put some ice and cold water in a bowl. Bring a pot of salted water up to a boil. Add the spinach, cook it for 2 minutes, and then transfer it to the ice water to stop the cooking. Drain the spinach in a colander, and then squeeze it with your hands until all the excess liquid has been removed.
In a medium saucepan, melt the butter over medium-low heat. Add the onion, garlic and 1/4 teaspoon salt and sweat the vegetables, stirring frequently, until they are soft but have taken on no color, about 6 minutes. Add the flour and stir to create a roux. Cook, stirring continuously to make sure it doesn't burn, for 3 minutes. Whisk in the milk and cook until the flour is completely dissolved and there are no lumps. Turn down the heat to maintain a low simmer and cook for 3 minutes. Whisk in the cream, return the mixture to a simmer, and fold in the spinach. Stir in the nutmeg, cayenne and 2 teaspoons salt. Cook for 2 more minutes and season with additional salt. Keep warm.
For the onion rings: Heat about 3 inches of oil to 375 degrees F in a large, wide heavy-bottomed pot over medium heat. Whisk the flour, Old Bay and baking powder in a medium bowl. Slowly stir in the club soda until smooth. Working in batches, dip each onion ring in batter to evenly coat. Fry the onion rings until golden brown and crispy, about 2 minutes. Drain the rings on paper towels and immediately season with salt.
For the steak: Put a large pan over high heat. Allow the pan to get very hot, almost smoking. Pat the steak dry with paper towels. Sprinkle the meat on both sides with the salt and pepper. Add some oil to the hot pan and carefully add the steak. To ensure a good sear, do not move steak in the pan. Cook for 4 minutes, flip, and cook for another 4 minutes for medium-rare. Let the steak rest for 3 minutes before slicing.
For the sandwiches: Add 1/4 cup of warm creamed spinach to the bottom halves of the Kaiser rolls. Slice each portion of steak in half, or desired-size pieces, and put them on top of the creamed spinach. Add a fried egg to each sandwich and top with Kaiser roll tops. Serve with onion rings.