Begin by washing and drying the produce. Thinly slice the scallions, keeping the white parts separate from the green parts.

In a medium pot, heat a generous drizzle of cooking oil over medium-high heat. Add the white parts of the scallions along with the bok choy and napa cabbage. Cook while stirring occasionally until the vegetables are browned and softened, which should take about 3 to 5 minutes. Season with salt and pepper, then transfer the mixture to a plate.

In the same pot, add another drizzle of oil and increase the heat to medium-high. Add the ground beef, breaking it up into smaller pieces as it cooks. Continue cooking until the beef is browned and fully cooked, approximately 4 to 6 minutes. Season with garlic powder, salt, and pepper, then transfer the beef to a plate.

To the pot used for the beef, add the Szechuan paste, Sriracha, peanut butter, half of the soy sauce, 3 cups of water, and 1 teaspoon of sugar. Bring the mixture to a boil, then reduce the heat to low. Whisk everything together until well combined and cook, stirring occasionally, until the sauce thickens slightly, about 2 to 4 minutes. Taste and adjust with more soy sauce if desired, and season with additional salt and pepper if needed.

Increase the heat under the pot with the broth to medium-high and bring it to a boil. Once boiling, add the udon noodles, using tongs to separate them. Cook until the noodles are tender, which should take about 1 to 2 minutes.

Divide the broth and noodles into bowls. Top each bowl with the sautéed vegetables, cooked beef, green parts of the scallions, and a sprinkle of sesame seeds before serving.