Marinate beef: In a medium bowl, whisk together all ingredients except for sirloin until combined. Add sirloin and toss to coat. Set aside.

Make egg garnish: In a small bowl, whisk egg and yolk with a pinch of salt. In a skillet over medium heat, heat oil and swirl to coat bottom of pan. Reduce heat to low then add eggs, swirling pan several times to make a thin layer. Cook for 2 minutes, then flip, cooking 1 minute more until eggs are fully set. Remove from pan, let cool, and slice into thin strips.

Cook the vegetables: In a medium bowl, whisk together soy sauce, sesame oil, gochujang, garlic, and a few grinds of black pepper. Bring a large pot of salted water to a boil. Add mushrooms and blanch for 2 minutes. Remove with a slotted spoon, pat dry with paper towels, and add to bowl with gochujang mixture.

To the pot of boiling water, add spinach and blanch for 15 seconds. Remove with tongs to a strainer and rinse under cold water to stop cooking. With your hands, squeeze out excess water and add to bowl with gochujang mixture. Toss to combine.

Cook remaining vegetables: Return skillet to medium-high heat and heat oil. Add red onion, carrot, green onion, and a pinch of salt. Cook until just beginning to soften, about 4 minutes. Let cool slightly, then transfer to bowl with gochujang mixture. Toss to combine.

Cook beef: Return skillet to medium-high heat and add marinated beef. Cook until meat is tender, stirring occasionally, 5 minutes. Set aside.

Bring pot of water back to a boil. Add noodles and cook until soft and chewy but not mushy, stirring occasionally, 5 to 6 minutes. Drain.

In a large bowl, toss noodles with sesame oil, honey, and soy sauce. Let cool slightly, then add in all vegetables, sliced eggs, and beef, tossing gently to combine.

Garnish with sesame seeds and more green onion before serving.