Remove any excess fat from the beef. Also remove the sinew, then pound the meat. Place it inside a Ziploc bag with the mixture from Step 1. Let it rest in the fridge for half a day.
Take it out 20-30 minutes before serving. Transfer the meat from the bag, and season well with salt and pepper. Do not discard the marinating sauce.
Slice the onion with a slicer, then soak it in water. Rinse the mesclun mix with water, then drain.
Toast the bread to your preferred crustiness. When done, spread the butter on one of the two.
Warm the frying pan over high heat. When it's hot, cook the meat quickly with the marinating sauce. Flip over when one side is nicely browned, and also cook the other side.
When the meat is golden brown, transfer it onto a plate with the sauce. Let it rest for a while.
Slice the meat after it has slightly cooled down. Please adjust cooking time to your liking.
Place some mesclun leaves on the bread, then top with the beef and sliced onions. Pour the sauce over everything, then sandwich with the other slice of toast.
Cut the sandwich into preferred sizes. Dish it up, and enjoy!
Story Behind this Recipe
My son said he wanted to eat bread with steak in it. I reduced the meaty odor with red wine, and then used olive oil to prevent the meat from drying up. I also added honey as I've heard that it makes meat tender. This mixture, also with soy sauce and fragrant grainy mustard, makes an amazing marinating sauce.
I usually do the prep the night before. I used imported meat, but I recommend you use Japanese beef because it's more tender.