Cut the ribeye into 1 to 1 1/2 inch cubes and season exuberantly with salt and pepper. Set aside to marinate for 5 minutes.
In the meantime, prepare the shimeiji mushrooms and scallions and start boiling 4-5 cups of water in a medium size pot.
Whilst the water is boiling, in a separate heated pan on high heat, swirl the butter until it is melted, slightly golden and sizzling. At this point, sear the ribeye in the hot butter, about 30 seconds on each side, turning constantly. Be careful not to overcook the steak. Once done, take it off heat and let it rest.
The water should start boiling whilst the steak is being seared. Once it is boiling, add the noodles and the vegetable mix from the packets and let it boil for ~3 minutes, stirring the noodles to let it cook until al dente.
When the ribeye is nicely seared and resting, the noodles should also be cooked. Drain away some of the water from the noodles reserving 1/3 of the liquid in the pot. Set aside.
Quickly sauté the shimeji mushrooms in the steak butter sauce for 1-2 minutes and set aside.
Put the pot with the noodles and water back on medium heat and add the seasoning packets of Jjajang sauce and Neoguri bonara spicy sauce. You can choose to add less of the latter if you have a less spicy palate. Stir well until the sauce is evenly distributed and has thickened slightly, ~ 1-2 minutes.
Once the sauce has thickened to the desired consistency, stir in the scallions and mushrooms and turn the heat off. Add in the fully rested ribeye on top of the noodles and serve immediately.