For the steak: Heat a cast-iron pan over high heat until very hot. Sprinkle both sides of the steak with salt and pepper. Place the steak in the hot pan, reduce the heat to medium and sear until well browned, about 4 minutes. Flip the steak and cook until medium/medium-rare, about 6 more minutes. Set aside, loosely covered with foil, and let rest before slicing.
For the sauteed garlicky spinach: Heat the oil and sliced garlic in a Dutch oven over medium heat, stirring occasionally until the garlic is golden brown and crispy. Remove the garlic and set aside on a paper towel to drain. Add the crushed red pepper and cook until fragrant, about 30 seconds. Add the spinach and stir to combine with the oil. Cover and let cook over medium-high heat until the spinach is wilted but still bright in color, about 4 minutes. Season with salt and pepper and toss with the garlic chips.
For the buttermilk blue cheese dressing: Mix the mayonnaise, buttermilk, blue cheese, dill, mustard and pepper together in a bowl and set aside. Adjust the seasoning if necessary.
For the sandwich build: Slice the rested steak against the grain into 1/4-inch slices, removing any non-rendered fat. Place the steak slices on the bottom bun, top with the spinach and smear the top bun with buttermilk blue cheese dressing.