How to Make It
Wipe any excess salt off kombu with a damp cloth. Combine kombu, soy sauce, and dried shiitakes in a small bowl. Cover and let stand at room temperature 24 hours.
Combine all ingredients in a medium bowl; stir until sugar dissolves. Set aside.
Sprinkle steak with salt, black pepper, and gochugaru. Heat a cast-iron skillet over high until very hot, about 3 minutes. Add canola oil, and swirl to coat. Add steak, and cook until deep golden brown all over and medium-rare, about 4 minutes per side. Transfer to a cutting board, and let rest 5 minutes.
Heat a small nonstick skillet over medium-high; spray with cooking spray. Break eggs into skillet, and cook until whites are set but yolks are still runny, 2 to 3 minutes. Remove from heat.
Toss together watercress, onion, and nuoc cham dressing in a medium bowl. Slice steak against the grain.
Divide steak evenly between 2 plates; drizzle each with 1 tablespoon umami soy sauce, and sprinkle evenly with furikake. Divide eggs and watercress salad evenly between servings. Store remaining umami soy sauce in an airtight container in refrigerator up to 3 months.