In a dry skillet over medium heat, toast the mustard seeds until they begin to pop and become fragrant, about 3-5 minutes. Stir frequently to avoid burning. Remove from heat and let cool slightly.

Once cooled, crush the toasted mustard seeds coarsely using a mortar and pestle or a spice grinder. Combine the crushed seeds with yuzu zest, salt, and pepper in a small bowl.

Pat the ribeye steaks dry with paper towels. Season both sides generously with salt and pepper. Rub the olive oil over the steaks, then press the mustard seed and yuzu mixture onto both sides of each steak to form a crust.

Preheat a grill or cast-iron skillet over medium-high heat. Once hot, add the steaks and cook for about 4-5 minutes on each side for medium-rare, or until your desired doneness is reached. Use a meat thermometer for accuracy (130°F for medium-rare).

Remove the steaks from the grill or skillet and let them rest for 5-10 minutes.

In a medium bowl, combine the softened butter, wasabi paste, chopped scallions, soy sauce, and lemon juice. Mix until well blended.

Transfer the compound butter onto a piece of plastic wrap, roll it into a log shape, and refrigerate until firm.

Slice the rested ribeye steaks and top each portion with a slice of the wasabi-scallion compound butter. Serve immediately with your choice of sides.