Make the horseradish sauce first. Bring the whipping cream to a light boil (use a bigger saucepan than it looks like you need, because it might foam over).
In a smaller saucepan over medium-low heat, melt the butter and stir in the flour. Add the cream to the mixture and whisk until thick. Add the other ingredients and lower the heat to low. Cook for another two minutes, stirring once, and remove from the heat. Keep it warm.
Season the steaks with salt and pepper or Creole seasoning. Heat the butter in a heavy skillet over medium-high heat until it bubbles. Put the steaks into the pan and sear them until they more-or-less unstick from the pan. (That doesn't always happen, but if the steak is really stuck down there it's probably not ready to turn yet.) Turn them over, tipping the pan to let to butter flow onto the spot where you'll put each steak down.
After the second side unsticks (this will take two minutes or so per side), transfer the steaks to the broiler rack in the oven. If you like rare steaks, turn the heat off. For medium rare, turn the oven down to 250 degrees.
While the steaks are cooking, melt the 1/2 stick of butter in a second large skillet over medium-high heat until the butter bubbles. Add the bananas and cook until they're browned--three or four minutes per side. Remove the pan from the heat and set aside.
Return the pan you seared the steaks in to medium-high heat, and add the red wine. Bring it to a boil while whisking the pan to dissolve all the browned bits and juices left behind by the steaks. After two minutes, add the beef stock or demi-glace. Raise the heat and bring to a full boil. Reduce the liquid by half.
Add all the other mushroom sauce ingredient to the pan. Lower the heat to a simmer and cook until the mushrooms are soft. Taste the sauce and add salt and pepper if needed. Keep the sauce warm while waiting on steaks.
Place the steaks on warmed plates. Top generously with the mushroom sauce, and pour the horseradish sauce on top of that in a thin, wavy ribbon. Set a banana half on each side of the steak, and serve while saying in a loud voice, "Behold! The world's best steak 'n' bananas!"