Marinate the steak and shrimp by whisking together the soy sauce, mirin, garlic, ginger, and sesame oil. Place the shrimp and steak in separate bowls or ziplock bags, divide the marinade between them, and toss to coat. Season with kosher salt and white pepper. Marinate for at least 1 hour, up to 8 hours.

Make the yum yum sauce by whisking together the mayo, tomato paste, sriracha, garlic powder, onion powder, rice vinegar, mustard, and white pepper. Season to taste with salt and store in the fridge until ready to serve.

Cook the shrimp in a large wok or nonstick deep skillet over medium heat. Add the oil, and once hot, add the shrimp in a single layer. Cook for 2-3 minutes per side until lightly browned and cooked through. Transfer to a plate.

Cook the steak in the same skillet. Add the steak pieces and cook for a total of 5-6 minutes until browned on all sides. Transfer to a plate.

Melt the butter in the skillet and add the onions and mushrooms. Sauté for 3-4 minutes, then add the carrots and cook for 2-3 minutes before adding the zucchini. Season with salt and pepper and cook until the veggies are tender.

Divide the steak and shrimp into bowls, serving with fried or steamed rice, sautéed veggies, topped with the yum yum sauce and scallions. Drizzle with Japanese BBQ sauce if desired.