Whisk together 1 tablespoon of the soy sauce, 1 tablespoon of the brown sugar, 2 teaspoons rice vinegar, 1 teaspoon of the sesame oil, ½ teaspoon of the garlic, and ½ teaspoon black pepper in a large bowl. Add beef and toss until well coated. Cover and refrigerate for 30 minutes or up to 3 hours.

Cook glass noodles according to package directions. Drain in a colander and transfer to a clean, large bowl. Add half of the sauce and toss to coat.

Whisk the eggs with ¼ teaspoon salt in a small bowl until combined. Heat 1 tablespoon of the avocado oil in a large nonstick skillet over medium heat. Add whisked eggs and swirl the skillet around until the eggs cover the entire bottom. Cook undisturbed for 2-3 minutes, or until the eggs are cooked through. You should have one giant circle of unseparated, cooked eggs.

Use two spatulas to transfer the eggs to a cutting board, being careful to keep it in one piece. Cut the egg circle in half, rotate the pieces 90 degrees and then cut into ½-inch wide strips. Add to the large bowl with the glass noodles.

Add the remaining 1 tablespoon avocado oil to the wok. Once heated, add the mushrooms. Cook for 2 minutes, stirring occasionally. Add the white parts of the green onions and the spinach. Cook for 1 minute, or until the spinach begins to wilt. Transfer vegetables to the bowl of glass noodles. Add the remaining soy sauce mixture and toss until well combined.

Return the wok to the heat. Once hot, add the marinated beef, taking care to shake off any excess marinade. Cook for 5-7 minutes, or until the fat has rendered off, and the beef is cooked through with a crusty sear. Transfer the beef to the bowl of glass noodles. Serve with the green parts of the green onions and toasted sesame seeds, if desired.