In a bowl, combine the soy sauce, dark soy sauce, dark brown sugar, coarse black pepper, cornstarch, and chicken broth.

Bring a pot of water to a boil. Cook the udon in the pot of boiling water according to the package instructions. Drain the noodles and set aside.

In a large skillet, heat the oil over medium-high heat and add the beef to the skillet. Sauté until halfway cooked.

Give the sauce a quick stir again and then add the udon noodles and sauce to the skillet. Toss to combine and cook for another 2 to 3 minutes or until the sauce has thickened to your liking.

Add the spinach and toss to combine. Once the spinach has wilted into the udon noodles, add the oyster sauce and toss to combine.

Serve as is or garnish with toasted sesame seeds and green onions.