In a food processor, pulse the onion, garlic and ginger for the marinade until minced. Add the soy sauce, brown sugar, mirin, garlic powder, onion powder, and black pepper. Process until well blended.
1/2 yellow onion, 1 Tablespoon fresh ginger, 1 Tablespoon fresh garlic, 1/4 cup soy sauce, 1/4 cup mirin, 1 Tablespoon garlic powder, 1 teaspoon onion powder, 1/2 teaspoon black pepper, 1/4 cup brown sugar
Pour in the water and sesame oil, and process for a few seconds to combine.
1/4 cup water, 2 Tablespoons sesame oil
Place the beef cubes in a bowl and pour the marinade over the top. Cover the bowl and place in the fridge for at least 30 minutes and up to 12 hours.
1 pound steak
Meanwhile, toss the mushrooms and onions in the olive oil, and season with salt and pepper.
8 oz baby portobello mushrooms, 1/4 cup onion, 2 Tablespoons olive oil, salt and pepper
Preheat your grill to medium-high heat. Once the meat is finished marinating, thread the beef, mushrooms, and onions onto skewers.
Grill for 3-4 minutes per side. It will depend on how big your cubes of meat are, and how done you like them.